(((((hugs))))))
I am glad you are sticking to your guns and putting your foot down. I think you did a good thing by pouring out your feelings in that letter to let your ex know how you really feel.
I managed to stay friends with my ex, but I wasn't married to him. We just always had a close friendship. I can truly say I have learned to open up and love again while still remaining friends with my ex, that's how you know you have truly moved on.
Good luck to you, and remember we are here for you through your grieving process.