Melminx - have just seen this
Hope you're bearing up ok. The news about his mobile bill isn't good. The evidence is certainly mounting up. It does sound as though he's got a lot of explaining to do.
I second all the advice given on this thread. Try to avoid doing anything rash, especially when you're in such a higly emotional state. Try to pin him down to a time to talk things through. Today's going to be hard - you feel all over the place and you don't know what to believe. Take a few deep breaths, cuddle your lo and try to put him and the issues to the back of your mind until such time as you can sit down with him and get his side of the story.
I think I remember that you've been in this situation before with him - is that right? Has he done this in the past? Did it go any further? If he has a history, then this is certainly the right time for you to make him realise that you won't put up with him treating you this way. It doesn't have to be a threat to leave - a separation of yourself from him can be in the mind as well as in the body.
Certainly start thinking about how you would cope on your own, maybe make arrangements to give yourself as much financial independence as you can. I know others have said this doesn't have to be the end of your marriage and I would support that but that doesn't mean you can't have a back up plan.
You can't expect yourself not to be a "pathetic jibbering wreck"! You're not that anyway you silly thing It won't hurt for him to see you cry but make sure you keep your position of strength - maybe say you need to take a break from each other (can you go to a friends/relatives for a few days or ask him to?), tell him you need time to think about where you personally go from here and so on.
Sorry I've jibbered on Hope you're ok and keep posting. You'll get loads of good advice and lots of virtual hand-holding xx