As I near age 47, prime time for mid-life crisis, I think about this a lot. It's a dangerous path to go down as too many regrets lead to depression.
So in a general sense I try not to regret, and instead understand why the paths I took were the paths I took. It helps a lot.
But if I could generalize, it is the fact that I generally did not believe in myself. Wasn't smart enough, wasn't good enough, wasn't attractive enough, "those other paths" were for "those other people, not people like me...."
I was just a poor white trash kid and therefore the ambition was to work in the town factory producing insulation.
As a result, I didn't participate in a single organized sport, including running. I didn't consider any college other than the local state school, didn't apply myself to my studies as I thought school was something to get through, nothing else.
Ended up not even going to college and took a half assed MOS in the Army because it had a 15k sign on bonus or something like that. Left the Army as soon as I was able and accidentally knocked my my wife on our wedding night, with no career and no money and no real desire to even have a kid.
But somehow, someway, I managed to find a job I actually love (Software Engineer) out of something that had been a hobby as a kid (programming). Somehow I make 150k a year with no formal education. My kid is a junior at Kentucky in the Honors Program on an academic full ride.