Crushing depression at knowing that I'm never going to be the person I always thought I was going to be. (And by crushing depression I don't mean clinical, just situational.)
If my teenage self could see me now he'd see a fat sellout who never did anything with his life. If I looked at him I'd see a stupid kid who didn't know nearly as much about life as he thought. Then I'd slap him around a little and tell him to go to fucking college instead of the Marines. I know I could take him. Unless he ran - he'd be much faster than I am now.