Sorry you have to deal with this disappointment. Bottom line, though, is that you have no choice but to accept it. Well, you COULD rant and rave and guilt-trip him and throw things, but in the end, if he wants to leave he will leave, and you will feel no better for having worked yourself into a frenzy of anger.
Living well is the best, er, I hate to use the term "revenge", but karma? You did your best throughout the marriage to deal with many difficult situations. You deserve a break. You are getting one--maybe not the one you hoped for, or expected, but this will not destroy your life. There are many opportunities to have a happy life without being married to someone who, for whatever reason, doesn't want to stay in the marriage.
Big hugs, you will come through this just fine, I predict.