Please help me. How can I stop being bitter/cynical?

[Vanessa]

Oh my. That is quite a long question.
I'm not going to pretend to know why your girlfriends tend to cheat on you. All I know is that you deserve better.

I understand that you're hurt - and that you feel so broken deep inside. Remember though, there is always someone out there for you. Don't give up hope. No matter how many people push you down, you must get back up. Your ex girlfriend's will probably end up regretting the fact that they left you. From your description, you seem like a great guy. Inside you, there is pent up frustration, and you must relinquish that in a healthy way. Keep in mind that not all woman are rotten - somewhere, someday, somehow, you will find the one that is right for you. You just have to persist - don't give up. Don't let one bad thing from your past affect your future. After all, it's not worth it.