I hooked up with a long-time friend who is also dating one of my long-time friends and I don’t know what to do

Two days ago I went to a party with a ton of my friends and everything was normal until my friend (M to make it easier) started hitting on me, heavily, and this was before we even started drinking, we were both sober.

Me and M have a small history together, it’s hard to explain but we’ve been close friends for over 5 years and there have been times when we liked each other at the same time but I either lost interest or nothing happened in general, but this was 2 years ago and since then she started dating one of my male friends (J). He’s not a childhood friend or anything but we’re somewhat close.

I really don’t have anything to say to defend myself for doing this, I knew she had a boyfriend and maybe I’m an asshole but when she started making moves on me I liked it and returned the energy. One thing led to another and we then went to one of the bedrooms.

Before anything happened I asked her something along the lines of “what about J”, and she told me to forget about him. I know it sounds bad but I didn’t really think about the severity of what I was doing to my friend. It’s even worse because I kind of have a girlfriend (we’re not really exclusive yet, though).

The day after I tried talking to her about it but she just told me not to tell anyone. It’s obvious she won’t but I don’t really feel good about this. I’m fine with keeping a secret if necessary, but if he finds out on his own, I will lose both a friend and my girlfriend

I’m sorry, but what the fuck am I supposed to do here? We’re all 18 by the way