[treme]

Two bandaid solutions for dealing with stress I'd recommend

Yoga Nidra - one of best ways calm your nerve system https://www.youtube.com/watch?v=_noquwycq78

EMDR https://www.youtube.com/watch?v=DALbwI7m1vM& it's going to sound crazy but just give it a shot. Just focus on the moving dot when feeling extremely stressed. Check out the top comments for rough how to guide, but just focusing to follow the ball alone will being immediate relief.

science behind it https://www.youtube.com/watch?v=xZVw-9ThmSM&t=31s

Long term solutions: Stoic philosophy... Imagining worst case scenarios playing out, and making peace with it. in your case, worst case is you fail and pick yourself back up working as software engineer with great salary. You'll manage ok.

Figuring out your stress-destress equation to figure out a sustainable balance of workload and sticking to it(I imagine very difficult to do in most startups).

Seeking professional help.