I’m confused on my ex relationship who is at fault?

I was dating someone in 2023 . Whe had just started dating when we got into an argument. He said it was over. he ignored me for a while. I was grieving the loss of my deceased mother. I was on a lot of drugs, crying because of a broken heart alcohol I made a mistake. I slept with someone. My ex says it’s cheating. When me and him were dating he had a sugar mama. I don’t know if they have a romantic, sexual relationship or emotional or whatever he never told me, but if he had any sexual relationship, flirting, romantic feelings, romantic actions or went on a date with her while he was dating me, that’s considered cheating too right?

I’m confused on my ex relationship who is at fault?
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