When I was in my 20's I would make the ultimate mistake of comparing my success/failures to my peers, people I went to high-school with, rivals etc... This is so counterproductive and can be completely damaging. People older than me would tell me that I need to run my own race and that it's not a sprint but I was too stubborn to listen.

If I had a time machine and I could go back to my past-self I'd try to slap some wisdom into my being.