Something has to change

I haven't been able to write anything for a few weeks. I have had something eating me up inside and haven't been able to talk about it or think about anything else. My grades are starting to suffer and so is my emotions. I found out a few weeks ago that my fiance relapsed. The only way reason I know about it is because I found it in his car. He has been lying to me for months about it. I knew he wasn't acting like himself, and he was constantly lying to me and was becoming the person he was 3 years ago. When I finally confronted him about it, he had no choice but to come clean with me. He said he was done and it wouldn't happen again. That was a lie. He is still acting the way he was and the lies continue. This weekend I went over to his house and he was with another woman. I know I should leave and not look back but I cant. I am head over heels in love with this man. He said he wants to go to counseling for him and also he wants us to go to counseling so he can find out why he has been treating me so poorly. We have been best friends for ten years and he said that I am still his best friend, but that's how he has been treating me. Like his friend and not his fiance. He claims he is in love with me still and has apologized for hurting me so bad. And he wants to make things right again with us. I want that too, but I don't know if I will be able to trust him anymore. And that hurts.