I think my bf may be cheating b/c of messages on cell. How can I confront him?

My bf and I have been together for a little over 2 years now. I had just gotten out of a relationship when we met and was a little wary of jumping into anything serious so we "took things slow" for for about 2 months. On my birthday we all went out as a group. I got a bit too tipsy and caught up with a visiting (female) friend and ignored my bf a little. We fought and I left. He then went home with someone (a friend of his) who has had a thing for him for years. They didn't have sex but he confessed" things" happened. We worked through this and decided to date exclusively.
It is now two years later. I am going through some tough depression and addiction issues at the moment and our relationship has been unsteady for a few months. I take full responsibly and regret every time I hurt him. We always work things out, are planning on moving in together very soon, and are talking about marriage. BUT...When I have my "off" times, we sometimes don't see each other for as much as 2 weeks. We also do not socialize together at all, he has his friends and me mine. Sometimes he will come out with me but I haven't been around his friends in a year or more. This was my choice b/c I embarrassed myself around them a few times. Last night we were at a book store and I asked to borrow his phone to call for take out b/c I didn't have mine. He went to pay for his book while I called. He has a complicated phone and I accidentally found myself in his messages. This was truthfully an accidental thing. Well I found one from this same girl asking him if he was coming out and then said "Bob is pouring your beer down the sink and I want you to come and do nasty things with your body to me". I was with him this night and he was getting txts the whole night with invites out (not just by her I know) But he was laughing at them and reading them to me, except this last one. I remember b/c I thought it odd. He just smiled and said "Oh god!", or "Oh my!" and I assumed he thought it was a joke I may be offended by. Apparently not. He hasn't acted differently or anything and if things did happen I know it's my fault for my "absences" but I'm terrified of loosing him. I also don't know if there is a way I can ask him about it without it looking like I snooped. I didn't act any different all last night but with a restless night of no sleep I could barely look at him this morning! HELP!
ps: Sorry this was so long!

She was being sarcastic with her comment.....she was joking.

Addiction wise, most people can do without a relationship when dealing with their addiction whether it be drugs and or alcohol. With all these troubles you seem to think that are going on in the relationship will only make you have a relapse. I suspect what you mean about your "absences" is your benders. You need to recognize that your addiction issue should take priority over this relationship or you will lose everything.