HN user
Can confirm. We use PIDs in espresso machines to control the temperature of the water used in brewing. Some machines have simple thermostats that over/under shoot ideal brewing temperature and a PID will help keep a consistent temperature or even let you override the temperature (if your machine runs too hot or too cold).
OP, this exact thing happened to me in high school then into uni. Well, I actually dropped out because I just could not make it work. I wasn't like the top students either. Hell, I was hardly even average.
If there is any advice I can give, it would be to be _proud_ of every accomplishment you achieve. People use pride to motivate them. The pride of finishing a course, an assignment, a homework problem, getting a paycheck, promotion, a diploma -- I could go on. I can't even count the number of times I thought I was prepared for a test only to end up with a barely passing grade (if I was lucky). Practice and preparation are empty, unfulfilling activities without pride, and I think that's why I could never get anything out of them.
Pride is something that I struggle with even ten years after I packed all my shit into my car and made the long and painful drive home from university. My failures still hang heavy on my every movement, regardless of the good things that I do today. It could have been different, I think to myself. I should have done better. It seems like you are struggling with the same emotions.
In the end many functional, successful people struggle with regret of _some_ kind. You seem to be remarkably introspective about yours, and that is a good thing. Let it guide you, motivate you. It took me ten years (and debt that I am still paying off) to trust those instincts.