I came across this paragraph a while back. It's stuck with me.
There are more things, Lucilius, likely to frighten us than there are to crush us; we suffer more often in imagination than in reality. I am not speaking with you in the Stoic strain but in my milder style. For it is our Stoic fashion to speak of all those things which provoke cries and groans, as unimportant and beneath notice; but you and I must drop such great-sounding words, although, heaven knows, they are true enough.
What I advise you to do is, not be unhappy before the crisis comes; since it may be that the dangers before which you paled as if they were threatening you, will never come upon you; they certainly have not yet come. Accordingly, some things torment us more than they ought; some torment us before they ought to torment us at all. We are in the habit of exaggerating, or imagining, or anticipating, sorrow.
- Seneca's Letter to Lucilius, Letter XIII - On Groundless Fears
Most of what we worry about never happens. The rest is manageable when it arrives.