Fucked an ex-friend’s ex?

Alright story time cuz I need to get this off my chest. So two of my friends used to be in a relationship but they broke up a year and a half ago. I’ve been friends with both of them for about 9 months. The male friend (jay) thinks they’re going to end up together and has had strong feelings for her since they broke up. The female friend (Sarah) does not seem to feel the same way. She has always told me that she doesn’t want to be in a relationship with him again, and she has told me since that she was in love with someone else. They’ve both been sleeping with other people since they broke up, and Sarah has been fine with it. But they also hooked up with each other on and off in 2018.

Sarah and I had a falling out about a month ago. She moved to a different state around the same time that happened and we haven’t talked since, apart from me saying merry Christmas. Jay and Sarah are still really close and they talk every day.

I got super drunk NYE at a party and jay and I had been talking all week about how we needed to get laid. On nye when I was drunk, we decided that our friendship was platonic enough that it wouldn’t be weird if we slept together (pretty sure he was sober). There were like negative feelings involved. Jay told Sarah about it and Sarah is furious with me. She said I betrayed her to the fullest extent, because she is just now telling me that she’s still in love with him (which I’m not sure if I believe, but I might just be trying to justify my actions). But she’s not mad at jay at all.

I sent her a long apology message and I feel horrible. I didn’t think this would be such a big deal to her and I know now that I was too drunk to be thinking clearly. I know no matter the circumstances of their relationship now, I did the wrong thing and I feel horrible. But I’m also really kinda upset about how she handled it, by telling me NOW that she’s still in love with him.

Has anyone done anything with an ex friends ex before? How did you deal with it? I know I can’t do anything else now and I just have to wait until we both get over it but this sucks