The "Will this still be important to me in 5 years?" test.
Nearly (not entirely) all the things I've ever worried about haven't mattered in 5 years.
In hindsight the relationship didn't matter (I'm in a happy one now), the company didn't matter, I gave it my best and walked away slightly burnt out but intact and my health while mediocre now is still better than it was a year ago (it takes a long time to recover from some things, rather than stressing about what I was in the past I focus on been better tomorrow than today).
Including tight work deadlines, exam results (I just re-sat some of them back then).
5 years ago I hadn't started (and failed) a company, I was still with my ex in an unhappy relationship and my health was fine.
Also stoicism, some things are just going to suck until they don't, accepting that they'll suck but not forever genuinely helps (me at any rate).
Also lots of things that can wrong are fixable even if they do.
Some of this I think just comes with age (not inevitably perhaps), I certainly gained perspective on things that in the past I'd have struggled to cope with.