Jodie I am so sorry for what you are going through. You have already recieved some great advice and I am glad you came here.
I can relate to your situation in many ways, as I was dating an addict for 2 years... I recently (2 months ago) broke up with him because I was through with the lies. Although I did the breaking up, it still was the HARDEST thing I've ever done. I will tell you though, it gets better every day!!! There are still bad moments, hours.....but honestly with time I have been able to see the situation more clearly. I think that will be true for you too. Before, when I was with him, I was so enmeshed with him that honestly I was not facing reality! Its so crazy to realize how sick
I was!
Some things that helped me were journaling (a ton), reading books like codependent no more and women who love too much, going to al-anon meetings, praying the serenity prayer, praying prayers of gratitude (can be very powerful!), getting out of my apartment, spending a lot of time around others so I wouldn't sit and think and cry alll day... I even posted some sticky notes beside my bed saying things like "I am good enough", "Whatever is meant to happen will happen", "I deserve love and respect", and "I will be happy again." Affirmations like these can be really helpful sometimes.
Otherwise, like others have said, try not to think too much about why or how he did what he did. It just doesn't make sense. The behaviors of addicts cannot make sense to us..it is useless trying to figure them out. It seems as though my ex has already moved on. I was devastated at first. It still makes me very sad and uncomfortable, but having no contact and realizing that I cannot control it makes me feel better. I do pray for him even though I am very angry at him...I pray that God will watch over him and protect him.
Hang in there, take care of yourself!!!