Hi Erin,
At the risk of arousing disagreement, I have to differ with one of your comments. You do have a choice as to whether or not you want to remain involved in this relationship. You can walk away. The real question is how much deception can you handle? Alcohol and drugs are notorious for warping the user's mind, which makes lying to ourselves and our loved ones just a daily exercise, used to further our own agendas. I can't even begin to count all the different forms of lies I've used to cover money spent at the bars. Lying just seems to go with the territory of being a drunk or drug addict. Furthermore, sometimes we will say anything to lessen the amount of hurt we cause, even if what we say is untrue. That's the real nature of this beast we are fighting with. If an active alcoholic tells you that he/she doesn't lie, beware. I could write a book of the ones I've used over thirty years. Now, that I think about it, it would be a comedy, because the only one I was really fooling was myself. As far as the infidelity is concerned, that's a tough one and I'm not really sure what path is best to take. Only you can determine how much of what he says is believable.
I wish you well in trying to sort out this most difficult dilemma. keep coming here for support and you will find your way.