I don't know if I should take my mom up on the offer to come visit, at least for a little while.
YES!!! Get your mom here asap. You will feel so much better.
DH asked to sit down and talk this afternoon, which somehow ended up with him being angry with me. I told him that if he wanted to be in a relationship with this girl, he needed to move out - it wasn't fair to me to expect me to be in the same house as him while he's with her. He got mad at first and said that wasn't fair, it was his house too and he should be able to do what he wants, but he finally agreed to leave at the weekend. We'll see if he really does or not. He also got mad at me for keeping the texts from this girl - I said I was doing it because some of them were starting to get threatening and I wanted proof in case I needed to get protection in the future. He said that was absurd, that she was "just a kid" (SERIOUSLY??!!

) and that she would never want to come near me. I told him if he didn't want me to get the messages, he should probably tell her to stop sending them and he got mad and said I wasn't listening to him. OK then. At that point we were done - now I'm in the living room watching TV and he's in the guest room sulking.
I wouldn't tell him any more about this. Unfortunately, you are now on opposite sides. Don't show him your good cards. Just in case. It takes a long time to finalize a divorce. The person he is right now is NOT the person that will show up for the final hearing. (You will not be either. But that is not a bad thing.)
I also wanted to comment on something else - people had been asking how our relationship was before this all happened. As far as I knew it was fine - we'd been blissfully happy the first couple of years together, then just settled into being happy and comfortable together. I honestly never had a clue DH was unhappy - he says he'd been unhappy for a few months when he started the affair, but didn't know how to talk to me about it. I've thought back over that time, and he honestly seemed fine - I actually looked at the pictures we took on a vacation together two weeks before the affair started, and we look so happy! But he can't have been happy, otherwise he would never have started the affair.