How do i stop this toxic cycle with me and my ex?

My ex and I have been broken up for over a year now. We’ve been friends the entire time, but it has turned into a horrible cycle where we both end up hurting each other.

For example, I used to lie during our relationship because I was extremely scared of his reactions to certain situations. Recently, I did the same thing again. He became extremely upset and disappointed, going off on a long rant about how he’ll never be able to trust me.

On the other hand, he ended up hooking up with the person he told me never to worry about while we were together. That just keeps replaying in my head; it makes me feel horrible every time I talk to him because that was the one thing I begged him not to do.

It feels like we are stuck in a cycle where we have the most amazing time together, but then it gets ruined because one of us ends up hurting the other. I don’t know how to stop this because he is the only person I have to talk to in my life, and I don't think I can live my life without him.