So my friend convinced me to be this girl's rebound guy and I managed to get a fair amount of action within the span of a month but when we talked half of the conversations were about how neurotic her ex-boyfriend was which was a little irritating but at the same time I could relate. A few months later I left town for good and I sent a random text trying to cheer her up then she implied she was trying to get back with the guy but sound like she failed and I said she has one-itis which she agreed. Two months later I get a facebook update saying she crawled back into a relationship with Mr.neurosis.

Why the hell would a girl go back into a relationship with someone after explaining a fair bit on how neurotic they are? If I vented about how much an ex was a bitch I would realize shes not worth anymore time and energy.

I was suggested that if I wanted revenge I should just be nice and act like nothing happened which would result in a big mindfuck. Is this true?

Ah, the mythological ex.
Ever happened to you that you saw a couple that the guy was so fucked up and his girl was so hot? and you couldn't understand how she doesn't dump that douche?
Women are so complicated that they actually think that they can get into a relationship and fix their guy, which almost never happens and always fails.
Their thinking is irrational, and act only by emotions, no matter how much they talk sense into themselves.

I can't see why there is need to "revenge" from your side - just move on and freeze her out.

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"Despite all the giggling, blushing and talk about having a deep connection, it all ends
with a cock in her mouth." ~60 Years Of Challenge