I screwed up....bad

My boyfriend (now ex) and I had been together for almost 5 yrs. We had many ups and downs which included him stepping outside of the relationship emotionally and cyber-sexing females. He said he did so because I didn't give him the attention be needed to be happy. I started to lose myself as a woman when I became pregnant which is when he started having sexual convos; we weren't ready for a relationship he did not want to let the females go once we got together. So yes, I did start to emotionally and mentally check out early on but we worked on it. We were essentially off and on for a few years. He went through a major surgery and went through depression. Around the same time the flirtatious convos with my supervisor grew. When I realized how far it'd gone I tried to pull back and stir the convo. I felt wrong and I knew it was wrong and when I hot the point where I was not doing it anymore, I stopped. Well after that I decided to walk away from my relationship. I saw that I was weak and naive and stupid by letting somebody in that did not have good intentions. I regretted it ever since. Today I told my ex about it and he flipped and was devastated and wants nothing to do with me outside of our 3 YO. I love that man beyond belief but I know I can't be on a relationship with him. We were gonna try to work it out down the line but I know that ship has sailed. How do I even get past this? I feel like he's putting more on me for my mistakes and he won't forgive me.