His moving on may be the best thing for all involved. Sometimes our gifts come in damaged packages.
People change. People change. People change. You have changed too I guarantee it from the woman he met 20 years ago, and had alcohol not been an issue you still would have changed and so would he. Like a relapse, a decision to leave a long term relationship doesn't happen overnight. He didn't just wake up and think "hmmm I think I want out of my marriage".
I find his comment that your marriage would cause him to drink to be about as low as one can go - what a weak assed man to put that on you, and its utterly untrue. If that's what he really thinks expect relapse because that kind of comment is very indicative of relapse thinking.
You will trust people again - as you recover from his betrayal and watch new gf and he sail off on the Titanic you may be glad it isn't you anymore. I hope so.
I like what Pippi said and how she is handling her situation - there WERE good times, there WERE amazing things, she got 4 wonderful children from him and no regrets. Yep, moving right along to greener pastures.