Certainty is one of the things that puts our thoughts to rest. Unfortunately, there is always some uncertainty about the future: it's uncertain, to varying degrees, what will happen in those parts of your life that haven't yet unfolded. You will usually have some expectations that are rather reliable and others on which you wouldn't bet; in addition, many things will happen that you don't expect at all. Among the things that in fact will happen there will inevitably some you wouldn't choose (if you had a choice). Fate can always be presumed to have some nasty surprises in store (and also some nastiness that won't be surprising at all). That's just how things are. But since there's no certainty, it's not easy to put your thoughts about those possibilities to rest. This is one of the sources of fear.
(There is also a kind of fear about things that are certain. The most extreme example is fear of death, but we also know a fear about smaller things we expect to happen, yet wish that they won't — an unpleasant meeting scheduled for tomorrow, for instance.)
Fear is a response to something you think might go on in the future, something you take to be bad for you. Once you look closer, however, you'll find that almost every fear you experience is about externals. And for externals, particularly in combination with uncertainty, there is an alternative attitude available: caution.
Fearing things in advance is foolish and only makes you miserable; caution supports you and makes you successful. Fears tend to self-perpetuate; caution satisfies once it is exercised. They even differ in their affective quality: fear feels bad, while caution feels good. It's not just that fearing things in the future simply isn't good tactics, whereas caution is clever management of the options that you already have in the present to influence those things to come. It's also that every time you act, with some aim in the future, you add a little to the subtle imprint that your actions have on your personality. Acting from fear, you're sending yourself messages that those external things in the future are worth getting emotional about; acting from caution, you observe yourself as balanced and in charge as far as it is possible. (There is a close analogue to this choice between fear and caution, with respect to the favorable things: if something seems desirable or preferable, there's an equal choice between an exuberant joy in advance and calmly taking all reasonable measures to make it happen and subsequently enjoying it.)
Caution, in contrast to fear, has the additional advantage that it puts your thoughts to rest. Caution means to take appropriate action and forget about the thing (maybe put a reminder in place to review your position when it has become imminent); fear means to bathe in the feeling that you have when you imagine the thing coming, and do it over and over again. Ironically, people often don't even take appropriate action to prevent the feared thing from happening or to mitigate its impact. They act as if having a feeling were a suitable replacement for effective action.
Fear and caution have in common that they are future-directed attitudes about something that is broadly to be avoided: they are attitudes aiming at prevention. But fear is an emotion: it construes that which is to be prevented as bad for you, as something uncompromisingly to be avoided. And as with all emotions, once it gets rolling, it is hard to stop. The fearful thoughts will cycle in your head, and whenever your imagination comes up with an additional detail, the intensity increases. And notice that this imagined experience is an extra pain that you create for yourself out of nothing at all. To anticipate pain or unpleasantness means that you make yourself feel it twice: once in advance, and then again when it arrives. It's not a good use of your powers of imagination if you employ them for making you feel pain before it is even there. With caution, on the other hand, the thing to be prevented is coolly regarded as undesired, dispreferred — but not as bad in any deeper sense. You don't take it as a danger to your person. (You know you can still treat it as a danger should it in fact become imminent). It's an external, after all, and this means all it deserves is caution, but not fear. Take care.