There is a widespread, but vague, feeling that life is short. What do we mean by this?
Sometimes lives are cut short by accident, illness, or even other humans' violence. In these cases, to say that a life was short means to say that it was shorter than it could have been, hadn't it been for these events which ended it prematurely. Of course, this is still very vague. (If an accident hits you at old age, is this still a premature death?) But it gives us a hint: the notion that life is short is an idea of a comparison; it's the idea that life is short compared to some other, longer stretch of time — a longer stretch that could have been the duration of life (if life wasn't, in fact, short).
Often enough, this idea rests on a simple self-deception. It sets in when people realize, at some point in their life, that already a substantial portion of that life has gone by. Sometimes this happens when we have grown old and physical strength and mental abilities, such as the ability to concentrate or remember things, begin to fade away. Sometimes it happens at a special date (such as an anniversary) when we look back over a segment of our life. When that happens, and we notice that we haven't made the best out of that portion, we might draw the wrong conclusion and think that what is missing is more time, not a change in the use we make of that time. We think that we're doing all right, but that we haven't been given enough time. Even though we realize that perhaps we have wasted some of it (waiting for another day, walking the path of least resistance, having some easy fun instead of doing something worthwhile), we think we're entitled to that. And instead of recognizing how important it is to make good use of all that time, we think that we've been dealt too little. Focusing on the duration also seems to put the focus on the most unimportant thing: on the 'when?' question. You won't find out, even if you think long and hard about it. But when you do that (think long and hard about it), you're wasting time again, subtracting from the amount that is in fact available to you. It's quite common for people who fall into this trap to carry on wasting time. (Which only makes sense given their idea that it's not them who should change, but whoever deals them their share of life time should have given them more of it.) They ignore the temporary glance they've taken on what's really going on, and fall back into merry self-deception.
At other times, it comes from a reluctance to decide and commit yourself. ("I would like to do this or that with my life, but I can't really choose.") If the result is that you do none of them at all, then it's not the shortness of life that is to blame for the fact that you weren't able to do even one thing. The real reason was not that life was too short, but that you didn't decide what to do with it.
In yet other instances, it rests on a fear of death. But letting a fear of death convince you that life is short would be foolish. In a nutshell, that is because there is nothing that you can do about death. What you can do is live wisely, but that is not about death, it's about living. Perhaps you have a certain influence on timing: you can be attentive to your health, for instance. But then again, where does this strategy come from? If you are attentive to your health just because it prolongs your life time, as if more time were the best (even a good) thing to have, you haven't picked a very good reason. Treating your health as an all-overriding goal is not a useful general rule, for frequently enough there are more important things than your health, at least things that might be considered more important in certain circumstances. On the other hand, if you learn to decide when your health needs some attention and when other concerns are more important, you have found a way of managing that aspect of your life; and this means simply putting appropriate care to your health, as far as that's the reasonable thing to do. But if it results from such a reasonable stance, then it doesn't result from fear of death, does it? You may have started from a state of fearing death, but that's neither necessary (you may have overcome your fear of death independently from that health thinking) nor is it really the cause of your getting to the reasonable conclusion. Once more, good choices come from making the reasonable choice, not from fear. And thus if you find yourself concluding that life is too short from a fear of death, then you're looking at a mistake of thinking, however plausible it may seem to you, supported as it is by the drive of the fear emotions.
Whether it is an inclination to waste time, a difficulty in taking decisions, or fear of death — none of these are necessary: you can always change yourself gradually to get rid of them. Remember that the opposite of a fulfilled life is not a short life, but an empty life; the opposite of a successful life is not a short life either, but a life which you failed to make into your own. Nobody can choose how long they will live, but everybody has a chance to make something out of their shot of life. Take care.