I'm sorry to hear about this, I can imagine how painful this must be for you.
Were the two of you living together?
How old are the two of you?
How was your intimate life?
As much as we think we know what's in another persons heart and mind, the fact is we don't. Every once in a while someone catches us with an open heart and shows us the worst of themselves. Obviously he wasn't as happy as he was making himself out to be and for all you know he's lied about this from the beginning. It's important for you to realize, right now, that this has nothing to do with you. I know you're saying, 'this has everything to do with me!', but honestly, when someone is dishonest, cheats, lies, it rarely has to do with the person they are doing it to but has to do with something within themselves. For all you know he did the same thing to the person he was with before you.
I'm not saying that you shouldn't talk to him, tell him the same thing you've told me....'what if you were really meant to be with me and now you're destroying that?', and see what he says. BotXXXXX XXXXXne, you can't make someone stay if they don't want to stay. You can't make someone love you when they obviously can't love you as much as they say they do or they wouldn't hurt you like this.
If he makes a decision to be with this other person, you're going to need to refocus your energy onto yourself and away from him. Take a class, teach a class, go on a trip, take up a hobby, or best of all do some volunteer work...there's nothing better to help yourself heal than to do for others.
I'm not sure if this is what you wanted to hear, or if i covered what you wanted to talk about....but I welcome your thoughts, and am certainly here if you want to talk more