Why can’t he let go of things ?

My ex broke up with me once in the past and during that fight I called him a “piece of shit” I never meant it and I apologized for what I said. Never did I repeat it again. Recently we broke up again and he still brought it up to me how he should’ve just ended it since the first time I disrespected him. Through our whole relationship we both have said things that are hurtful out of anger and pain but overall once talked about and apologized it would be okay. At least that is what I thought. Turns out it seems he was always holding a grudge. The reason why he broke up with me the second time was because I was pressuring to talk out a fight during a long car ride home and he said he felt abused. I apologized for that but there was a lot of alcohol involved that day.

Couple days later I asked him to work things out and he said “me going back to you is like you going back to your ex who hit you”

How can he compare !?

Our relationship was mostly good and harmonious. Full of love and support with minor clashes like any couple has

Why is it that he couldn’t let go and why is it that due to us having another fight he threw everything away. I accept it now but still continue to feel so confused.