I have a girlfriend, but I'm dating someone that isn't my girlfriend.
Why would you do that? Isn't your girlfriend going to be upset about this?
Fuck her. Sometimes I think she's cheating on me, but that's why I'm also in the relationship because I want to see her unfaithfulness in person
It does not sound like you are in a very healthy relationship to be honest. If you think your girlfriend is cheating on you, that does not give you reason to cheat on her. What it does give you reason to do however is to talk to her and be open.
I'm done here.
I'm only trying to give you advice. If things are that bad and toxic already the best thing for you would be the end things. That way you both don't have to hurt each other.