It wasn't about you. There is nothing great about her. There is some post on here called "they always trade down" and it is so true.
This is his story. His problem. This isn't a rejection of you- this is about his insecurity, his problems, his low self esteem. It may not even be about your marriage.
Where do you go from here? You need to step away from it, don't talk to him about it and think about what you WANT. Is this someone you think you want to be with? that he could do this to you? Is it a dealbreaker? If you get him back, what are you really getting? Is it worth it?
Time will tell. You can go to MC, IC, whatever. You are in control now...YOU DECIDE. not him.