I think I'm a bad guy
Why do you think that you're a bad guy?
Yeah, I'm really bad. I do all kinds of bad things to my girlfriend...
If you feel like you have been acting badly towards your girlfriend you should try to amend your ways. It's wrong to mistreat others. If you think you can't make those changes alone, enlist the help of a couples counselor in order to constructively work on your relationship with your girlfriend.
I won't talk to my girlfriend about these things, so I guess that wouldn't help.
You're right, it wouldn't. Another option would be to see a therapist on your own, to process the reasons you do the things you recognize as bad, and find more constructive ways of interacting in a relationship. This could help you have a better view of yourself, and likely improve your relationship with your girlfriend.