I found out that my ex kissed and gave a handjob to one of her friends

I found out that my ex kissed and gave a handjob to one of her friends. We don’t talk anymore and we’re not in a relationship, so I know I don’t really have any right to be upset about what she does now. But honestly, hearing it made me feel so fucking low.

I had always had a doubt that she might have cheated on me when we were together, but I never had any proof. So hearing this now kind of brought all those old feelings back, even though I don’t even know if she actually cheated back then.
The confusing part is that I’m not innocent either. I got blowjobs from two different people and I also started liking someone else while we were together. So I know I did things that would probably hurt her too.

That’s why I don’t even understand why I’m feeling this bad. I don’t want her back, I don’t want to control what she does, and I genuinely thought I didn’t care anymore. But since I found out, I’ve just had this horrible negative feeling that won’t go away.

Maybe it’s jealousy, maybe it’s my ego, maybe it’s because I’m realizing she’s moved on, or maybe it’s because it made me think about everything that happened between us. I honestly don’t know.
I just feel really lonely right now and don’t really want to talk about this with anyone who knows me, so I thought I’d post it here and see if anyone can help me understand why I feel like this.