Why is my ex husband sending me insulting letters all the time?

For 10 years while we were married he ignored me, said that he didn't want to make any effort to improve our relationship, and seemed happy when I left. Now he started dating one of his pretty young college students and all he wants to do is write me letters telling me how horrible I am and what a bad wife I was. I'm really hoping this is just a phase, but, aarrgh!! I LEFT BECAUSE IT SEEMED THAT HE DIDN"T LOVE ME. WHY - NOW THAT I'M GONE - DOES HE NEED TO GET EVEN WITH ME???? He's even planning on marrying her at the ski lodge we got married at. Help!! Why is he doing this?