The Fading Affect Bias
The goodness and badness of memories fade over time, but the badness fades faster.
Some bad memories even become good memories, while good memories rarely become bad memories.
It makes sense that both joy and pain fade with time—stuff just feels less intense when it's farther away—but why does pain fade faster?
It's because when bad stuff happens to us, our psychological immune systems turn on.
We start to rationalise ("Why would I want to be with someone who doesn't want to be with me?"), downplay ("Breakups happen all the time in high school, it's no big deal"), distance ("I never liked her that much anyway!") and distract ("I'm gonna go play video games").
These mental processes function like emotional antibodies, taking the sting out of bad memories.
We don't use them on good memories, so good memories keep their lustre longer.
Everything is temporary, bad stuff especially.
"Tragedy + time = comedy" is the closest thing psychology has to a chemical equation. ~ Adam Mastroianni
So even when things are going badly, know that in future you'll probably be able to laugh about this.
Discomfort in the present can be very painful, don't make it worse by fearing how you'll feel about it in the future, you'll be fine.