I cheated an I don’t know why

Quick backstory- I was with my ex for 5 years, met this guy at uni and became best friends and I fell in love with him. I then broke up with my ex because of that, then about a week later me and the guy I love kissed and confessed our feelings. We went on dates, it was absolutely perfect. We were basically a couple. Then one night I got horrendously drunk and got with another guy basically in front of him. I begged and said I would change, it was so out of character for me. He forgave me, gave me another chance and then a week later we became official. Then 10 days later we were drinking with some friends and I for some reason kissed this girl whilst he was in the bathroom. I don’t know why. He said he’s giving me another chance because he can’t live with our me but I’ve broken him and damaged the relationship.

I Know I’ve fucked up so badly. I know I’ve hurt him so much, I just don’t understand why I push the people I love the most out of my life and continuously hurt them. I don’t know what to do, I can’t even trust myself to not do it again. I hate myself. I’m getting therapy soon which will help and I’m cutting down on alcohol massively. I just don’t understand why I would do this to someone I love so much and makes me so happy.