I hear ya. Grad school did a number on my mental health. The fear of failure is embedded in just about everything, from the high GPA expectations (even from your first semester), to the quals, to the thesis. The pressure on yourself can be too much to bear.

I find myself less stressed in industry having worked in various positions over the years, and despite all the college I have yet to use calculus at all (it's spreadsheets as far as the eye can see) and rarely have to use any of the other number of subjects I took in college. The one school course that had helped me the most in my actual work career has been my middle school typing course, I can type super fast as a result lol. Maximum productivity with my mechanical keyboard at work, keycaps go brrrrrrr.

I used to feel ashamed at myself for not going all the way, for withdrawing soon after I got my masters. I felt like a complete failure. This has lessened over time as I realize that the entire rest of everyone's working life is a grind regardless of how far up in academia you go, and that I would be in a very similar position otherwise.