Why :(

So. I have been with my boyfriend for 6 years...he came home last week and told me he has been sleeping with a girl from work. He got an anonymous message from someone saying that if he didn't tell me they would. So he told me. He didn't tell me many details. He said he didn't love her, he would never want to be with her properly but it has been going on for about 4 months. I didn't really get angry. I don't know why. I guess I knew something wasn't right. He said he knew he had to leave and that I would never forgive him. He said being with me was the best thing that ever happened to him and I asked him not to leave.

Over the next few days more details came out through speaking to his friends. He has tried to end things several times and she had threatened suicide. She even went as far as trying to In front of him once. He felt like he had already ruined us and was now trapped with her. When he told me he was broken. He was honest with me...he told me everything he had said to her to stop her doing something stupid. Told her everything she wanted to hear.

Anyway I was on my way to work the other day (she knows the route I walk -I have met her many times and she always tried to be my best friend. In fact the most recent time she suggested she I a go on a weekend away together. Now it seems like she was just messing with him...making it clear she could tell me if she wanted to. She stopped me on the way to work and said she wanted to apologise to me for the fact that my boyfriend is in love with her. When I told her I knew he had told her (implying it wasn't true) she started to cry and saying that men always treat her this way I snapped at her and told her to stop feeling sorry for herself. Immediately the tears stopped. I walked away. She shouted at me that she would never leave him alone. When I got back home my car had been keyed. She has been blocked on everything by both of us. She is no longer working with him as she had also been sleeping with two others at work and it all came out. I just don't know what made him go with her. I have been a mug accepting all of his excuses as to why he wasn't home. Every time. And now I have this crazy woman in my life who...I really don't know what she's going to do next.

I don't really know what question I am asking but any thoughts would be appreciated. I never thought this would be my life but then I guess no one does really. The betrayal is just so hard to take