Well my ex girlfriend broke up with me twice since then we became friends and now I think she likes me again. I have really never gotten over this girl... should I pursue her?

Yeah, why not? Just don't make yourself always available so she won't disrespect you again. Take it slow (for yourself) and she will realize that you are serious, and that you don't want to play any games.

What if she leaves me again?

Well mate, if she has already broken up with you twice I would highly consider moving on. Life will go on with or without the girl.
Also from reading your other post if that is the same girl find some one else. She sounds like she is not ready for a stable relationship yet. This explains the constant swaping of partners, you will find that this probably happens just as the relationship is getting "serious".