Remember all the bad times and all of her bad qualities. This helps if you actually had a relationship with the girl and it wasn't the "nice-guy" imaginary relationship where you want her but she never ever gives you the time of day.
Picture her f*cking other guys. She's moaning and screaming and the walls are shaking and you're in the other room. In all likelihood, this probably DID happen at some point when she wasn't reciprocating your affection. It's hard to imagine. You'll want to make believe that she could never do something like that. But the likelihood is that she did ride some other guy's dragon. Maybe when she "had to go" or she was "going to the mall with her friend", some guy was at her apartment f*cking her against every wall in there and on top the washing machine. Accept it. Picture it. Use the emotions you feel from picturing this as fuel to improve yourself and get hotter chicks and be your best self.
Realize that she just wasn't attracted to you and that attraction's not a choice. Don't be mad at her. Be mad at yourself for allowing yourself to be such a naiive pvssy. Figure out how to make yourself more attractive (by reading the DJ Bible and fastseduction.com) and do it.
And once you've improved yourself and made yourself more attractive, and if the urge so overcomes you, f*ck her friends.

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