I've been cheated on and I'm looking for some advice from women who have been cheated on or have cheated

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I've been with my boyfriend 3 years today actually. It's been a bumpy road but it always leads us back to eachother. I have a connection with him I've never had with anyone else. He's my boyfriend and best friend all in one. A couple months ago I found out he cheated on me , from what he said he made out with the girl twice. I never asked her because I knew I'd wanna kick her ass considering I beat the shit out of him. It took some time but I decided to stay with him and work through it. I've been fine and lately we've been happy doing more. But recently everytime we fight it goes back to what he did. I can't get over it at all. I think I do some days and other days I'm so invested in it I can't think straight or be happy with him. Everytime I ask why he did it he never gives me an answer For women who have been cheated on how did you overcome it andnfor women who have cheated (no judgment at all) was there a reason if you truly loved the person? I'm just looking for advice and maybe some answers. Thanks