This probably sounds weird, but I agreed with my girlfriend that I had cheated.
So did you actually cheat?
No, but for some reason I feel guilty, even though I'm innocent.
While cheating is wrong, if you did not cheat, you shouldn't feel pressure to say that you did. You should reconsider the kind of relationship you're in if you feel like you have to admit to things you did not do.
It just came up in a conversation so I said it. You think I should leave my GF?
If you're feeling guilty for no reason and confessing to acts you didn't commit, you need help. I can't imagine your your girlfriend is taking this. For both your sakes, make an appointment with a therapist.