You're welcome, of course. Keep battling. And if'n you can get help, privately or through school, get it. If it is OCD, it's chronic. OCD never goes away, but it is treatable.
And take it from a man more than twice your age, even if you HAD cheated on your girlfriend, that does not define the person you are. Nor would it define the woman who might cheat on you. Good people make mistakes all the time. Big mistakes, little mistakes, and everything in between. You're still learning who you are in your twenties, you're bound to do things you regret. I certainly did. Only now, I don't regret them. They're just things I did, some of them I'm now glad I did even.
You will not get through your life unscathed and pure. You will look at other women, fantasize about them, flirt with them, and your girlfriend will do the same thing with men, or women if that's what she turns out to be into. And it's okay. We are humans. We are evolved to be attracted to people. If you were never attracted to other people, THAT would be something to worry about.
Good luck, get treatment if you can, you're a good person. I know because bad people don't even feel guilty about bad things they've actually done, while you feel guilty about something you never did. You will make mistakes, but you aren't capable of being a bad person.