I dated a girl I didn't even like for about two months
I'm saddened for you, but why even date her if you did not like her?
I guess I hated myself for doing it
It's wrong to make someone think that you care about them when you really don't.  You could really have hurt this girl's feelings.  And if you weren't happy either, that was a clue that it was time to end the relationship.
I am a loser.
You may feel that way, but only if you continue to do things that you are not happy about and keep self-sabotaging, you will feel like a loser for a very long time. I hope you get better by talking to your friends and families, and better managing yourself and doing things that would make you happy and not self-sabotage, okay?