He tells me he felt so horrible about what he'd done and thought seeing her would make things better, he'd get some explanations, he says he really had no idea why he met with her
He met with her because he wanted to meet with her. Period.
They smoked pot and drank and then it was really awkward and she went home.
So, not only did he get f'd up and cheat on you with her.. he met her again, and got f'd up again. That makes absolutely no sense.
After a drunken/high one night stand, could seeing the person make things better? He says it was a way of a 'goodbye', he had no intention of ever seeing her again.
This also makes absolutely no sense. What did he need to say 'goodbye' to? Did he need closure (laugh..)? He saw her again to let her know he didn't want to see her again?
Why is she still able to "IM" him? He loves you to pieces, and it wouldn't have happened if he wasn't drunk.. but he's seen her since, drank with her since, and still gets messages from her. I'm sorry.. but I don't see how he's proved you mean more to him than her. That's probably true, but to be honest.. he sounds like he's still scrambling and juggling things. I wouldn't email her, that's totally misplaced energy. The person who hurt you (unforgivably in my world) is your husband.
I agree with the previous poster.. what you choose to live with, is your choice. What is ok in your marriage and in your world, is your choice. I hope he turns this around, and works his ass off on recovery and gaining back whatever shreds of trust that are left. Alcoholism doesn't cause cheating, cheaters do. Leave him to whatever he's going to do. Please try to take some time to focus on yourself, how you're doing with your life.. what positive things can you do just for you today? How can you take some steps to detach yourself from this situation? What are your plans moving forward?