Honestly dude it sounds like you know you did wrong and you aren’t going to do it again. You didn’t have an affair, didn’t fall in love, didn’t have sex, etc.. you had a five minute lapse of judgement while intoxicated. I know it seems like the end of the world right now but you’re young af and this is the first time you’ve felt these feelings. If everything else is as good as you say it is, there’s no need to fuck everything up. You learned an important lesson which is that you don’t want to cheat on this girl and you’ll most likely be more responsible about the situations you find yourself in moving forward in your life.

If you just can’t live with yourself if you don’t tell her, she may leave you she may not. The lesson is going to be the same, to not let yourself get into situations that lead to doing things you will regret with not your girlfriend. You may want to just stick with something like “this girl started grinding on me and for a split second it was exciting because I was fucked up and then reason kicked in and I rejected her because I would never want to compromise what we have”

It’s going to eventually be alright either way. You’re not a bad person, sometimes we learn the things we don’t want to do by how we feel after we do them. Good luck man.