Stay away from girls you have to fix or help them with their problems. Stay away. Most of your problems seem to stem from helping some girl and then having her leave you for someone else. There is this movie called The Last American Virgin. In it a guy is totally in love with the girl that his friend is dating. His friend (who is of course a total jerk) gets her pregnant and leaves her. The guy helps her through all her problems. He is kind to her, respects her, etc. At the end of that movie, she goes back to the jerk.
Girls like that will always do that.

I'm not trying to be mean, but you need to just focus on yourself. Learn to be comfortable with yourself without feeling the need to be with someone all the time. Stop comparing yourself with other people. Meet a healthy girl that doesn't need to be fixed. It will happen if you surround yourself with positive people and not those that are miserable all the time.