Serpentaura, I can truly empathise with the way you are feeling. I too have had regrets about decisions I have made in the past. I have had a few opportunities to pursue meaning relationships, but have lacked the confidence, courage, and absconded in fear. I am currently working on overcoming this.

About this girl, what attracts you to her? Did you ever have a meaningful relationship with her? Have you ever approached her and told her how you feel about her? If not, what has stopped you? Do you still see or talk to this girl?

The only good advice for overcoming these feelings of regret is to first and foremost, FORGIVE yourself. Accept reality, and know that everything you have done in the past is okay. Accepting the situation is key to put the past behind you and concentrate on the only thing we can change(the present and future).
Holosync, offered by Centerpointe at www.centerpointe.com can be very helpful in dealing with your current unhappiness, I have personally used it for over 6 months and it has been very emotionally cleansing.