You joined a while ago I see. Reread the DJ Bible for a start. Do it several times if you have the patience and time.
But some general pointers for you:
You are the Centre of Your Own Universe. Nothing anyone does should shift you from that centre. Only your own mind set can do that. Get positive, even though it may hurt.
Don't worry about others, worry about yourself. Get into shape, get a life plan/ambition and work at it, get a 'makeover' (doesn't have to cost much, shop around, use ebay).
Widen your social group. If someone flakes on you, then it's not an issue, you can more readily find other people to be involved with. If your 'friend' is so unreliable, drop them till they sort themselves out.
Again, you don't need to feed off others emotions and whims to validate yourself. Validate yourself with your own opinion and actions.
EDIT: drop the one-itis. Drop it right now. If you don't mind not being friends again/won't make your normal/common social situations uncomfortable, ask her out, if she's single. (if she has a bf, try to shut him down, then get a date) She'll accept, or she will turn you down. If you get 'let's be friends', turn it down ('I have enough friends thanks). Or find other girl(s) to ask out. You'll forget your interests in her with time.