There are inevitably in our lives what we might call losses: you accidentally put a book where you won't find it again; your baggage goes missing when you travel by plane; you suddenly find yourself out of a job; a romantic partner breaks up with you; your home is destroyed in a disaster; a loved person dies. Let's pick an example from the middle of this range for our discussion. (If we choose one of the most severe, emotion might easily cloud our judgment; if we choose too lightly, we may have difficulty to generalize plausibly.) Let's say, then, that you just found yourself unexpectedly out of a job.
What makes something a loss? For one thing, what you lose must be of some value to you. Perhaps there was a sense of fulfillment when you were working in this job, you were doing something that matched your abilities or potential; or the relationships with others on the team were quite good; maybe there were great learning opportunities or career prospects — something, at any rate, was in it that was of value for you. (Let's ignore for the moment whether it was, on reflection, something of real value or just an external. We're not talking here about whether it should be of value to you; we're interested in what must be the case so that we regard something as a loss, and that seems only to be so with something we in fact value, whether we do wisely so or not.)
Also, you must in the past have possessed or enjoyed that which you lost, and still remember it. If you have no recall, it's difficult to take it as a loss. Perhaps others might ascribe to you that you lost something, unknowingly. But it seems doubtful whether this really is a case of loss. (There is perhaps a special case with something like 'He lost his memories.') And obviously, that possession or enjoyment must by now have ended.
Is it still a loss if it was scheduled to end? If your job was on a contract that was made for one year, and this was clear from the beginning, then you wouldn't really have 'lost' your job when that year was over. So a frequent companion of taking something as a loss is a kind of comparison: we think that we lost something only in cases where we originally had expected (wished, hoped, fantasized) to keep it, but in the event our possession, enjoyment, or involvement was terminated instead. Losing something means not to have it anymore in fact, although we still might have it. Events took it away from us, but that was neither necessary (in the sense that it couldn't have been otherwise) nor expected.
This comparison, however, is seldom realistic. Things do get lost on travels from time to time; there are factors outside of your control that can make you lose your job (perhaps the company had a bad year and is forced to lay off people irrespective of merit); romantic partners change, as persons always do, and enter new stages in their life where they need a different kind of partner or no partner at all; and of course, all people will die sooner or later and there is no guarantee that it won't happen earlier than expected, from illness or accidents. Moreover, in many cases part of the responsibility lies with you, too. (There might have some neglect on your side, lack of precautions against things getting lost, or paying too little attention to your job or relationships.) In either case, there is always more or less of a chance that these things end; and if you ignore that possibility, not keeping some healthy measure of memento mori, this will lead to those unrealistic expectations which are part of the perspective that make you see things as losses.
What is the appropriate response to losses? It is commonly accepted that people feel distress, the intensity of which corresponds to the graveness of the loss. Distress and loss are entangled; you can't really get rid of distress while you keep thinking of something as a loss. But once you figure out that seeing something as a loss is an incoherent (if common) view of those things, it becomes rather difficult to remain distressed; viewing things appropriately will dissolve this kind of response. (Although other ingredients may still remain in the emotional mix even if distress is dissolved: shocks, phantom pain from habits, regrets and the like. We're only looking at one ingredient here.)
Distress feeds off the idea that you have suffered a loss. It starts with that comparison between the time when you had or enjoyed the thing in question and the present time when you no longer have it. This is an automatic means to feel bad, because that comparison will always make you seem in a worse situation now. (It's similar to the case of fearing future things in advance: a comparison with a foregone conclusion, a comparison already set up in a way that makes the present time losing out.) Consequently, you feel disappointed or sad, angry or frustrated, perhaps even desperate about it.
For although this is perhaps not obvious: it's not a comparison between the past and the present. It's a comparison between the actual present and some possible present you think of, which is modeled on a counterfactual continuation of what was there, or was the case, in the past. But as we have seen, this comparison is only sensible up to a point. (When we say of someone that he unnecessarily dwells in the past, this doesn't quite capture what's really going on. We should rather say that he dwells unnecessarily in a false version of the present: an unreal one.)
Then what alternative is there to seeing things as a loss? There is the stance that we might call 'remembrance'. Let's get back to the job example. If you had kept the possibility of a sudden firing in mind all along, you would be prepared and better able to face the situation. Of course, you might still feel some surprise and sadness now, in the actual event, but this won't be as gripping as actual distress, and far better manageable. You may even entertain, for a moment, thoughts about how convenient it would have been for you if you had kept this job. (Maybe it was well-paying, and you'll struggle to find another one that pays as well, or maybe the commute was short and pleasant, and it's clear that this will change now.) But again, only for a while — and then you get back to reality, to the present and the task now at hand, stopping well before getting into unrealistic comparison mode. You won't let yourself go down a path of denial, dreaming about possibilities that haven't made it into actual reality.
So whenever some preferable state in the past ends, we can recognize that the ending was inevitable (though we couldn't foresee the timing), and view it more coolly. We don't let distress take over by suggesting to ourselves that we might have had them longer. That is the comparison mindset I mentioned above: we compare what actually is the case with what might have been the case, find that we would have preferred the unactualized possibility, and seek to frame it as an injustice, of which we take ourselves as a victim. Remembrance, in contrast to seeing something as a loss, requires no such comparison and is therefore, even though it may sound paradoxical, a more 'realistic' attitude (than viewing something as a loss).
So the best way to deal with losses is to see them not so much as losses, but as contingent events in the past whose time structure is as little up to us as that of events in the future. For as long as we view them as losses, we build the way in which we suppose they affect us (as the ending of something that was valuable to us) right into how we view them. We've already fixed how we will feel about them, and after a while, it becomes a false sense of inevitability. Instead, find a healthy stance of remembrance: freely look back to the past. (To be free means here not to have an automatic emotional reaction such as distress, not to be a slave of your passions.) See the past as it was, but don't take it as necessarily required to categorize it as good or bad. And most importantly, stay in the real present, not an imaginary version of it: stop comparing reality with what might have been if the past had turned out differently.
Let me briefly summarize all this: when we look back at something in the past which we no longer have or enjoy, we can view it as a loss, and consequently feel distressed; but this perspective on something as a loss is not the only way of looking at the past, and it involves some self-deception; changing it may free us from distress, and will in itself have great value, too: as a more truthful way of looking at past things.
Whenever we look back at things that we can no longer enjoy, we have to decide which path we want to go down: one is the path of distress, where we view them as losses; we take it that something we valued has ended, and so we primarily focus on the more impoverished time now and ignore what was before. (In our example, that would be the experiences in our former job. There must have been something favorable in it, otherwise we wouldn't take it as a loss; there must have been something in it that made us look at it as valuable. But rather than focusing on that, we focus on what comes after.) When on that path, every time we recall the past, this will only trigger the ever same thought: that it is no longer here for us. (A certain self-pitying may very well be in play here, too: we pity ourselves for being off worse now for the loss.)
The other path, that of remembrance, is to look at the valued thing itself, as it was (and how it was for us) while it was there. This is of course rather difficult if we take it all as an instance of a loss. Once we leave that view aside, remembrance gets easier. We can see the valuable thing and the time it was here for us in a much more satisfying mode: the sense of fulfillment, having been able to do something that matches your abilities or potential, the good relations with others on the team, success and excitement, learning experiences and career steps, and more.
We remember the good time, and as a nice extra, in remembrance we can also be certain of it — nobody can take away from you a good thing you had in the past. No-one can take away from you how it was for you. To put it somewhat paradoxically: what from the perspective of distress is called a 'loss' is, viewed under the perspective of remembrance, the only thing of which it is guaranteed that it can't be lost. Nothing can destroy it (except loss of memory). Take care.