Is this worth saving or should I just move on?

I was with this girl for almost 3 years. We were engaged and planning on getting married in a few months. I'm in love with her still but I can't be with someone I don't trust. I was away on a business trip for a couple weeks while I was gone she started hanging out with an ex. She told me that they kissed a couple times. I told her that we could get through that together but then she informed me that she WAS going to continue hanging out with him. I told her that I wasn't okay with that because he is still in love with her. I told her that if she wanted to be friends and hang out with him then there was no way we could be together. Maybe I over reacted but she had previously told me that she has cheated on every other person she was with (which was a huge red flag). So I broke things off with her and moved out but I can't stop thinking about her and I want to be back with her. She tells me every day that she is sorry and she wants me back. But I can't get over the fact that she would throw our forever away over someone that is just a "friend"..So should we try and work things out? Should I move on and forget about her? If so, how do I move on when I know I'm hurting the person I care about the most?