Wow, I would feel threatened by this letter. I don't think I would be overly keen to hear this stuff said as it is. Its demanding and has no sense of consideration for his feelings... I'm not saying that you don't have a right to be very concerned and that I don't understand that you are hurt... I just wonder if there is a way you could use some "I" statements more? Its all you did this and you did that. It's all put on his shoulders and if he doesn't do as he is told then that is it? Geesh, harsh no?
I agree, some people are not cut out for the practice of poly and are more supporters of the theory rather than the practice... perhaps he is one of those, but I think that re-writing with a different perspective that comes from your feelings and asking him how he feels might help also. I dunno, maybe he is the type that would jump to the occasion and do everything you say... chances are, even if he did, he could fuck it up again as the root of the issue is not really addressed and he has no way of having his needs thought of... hmmmm. ya, I personally would have a hard time with this ultimatum.
I suggest doing some reading on non-violent communication. There are a lot of websites and a book by the same name by Marshal Rosenberg. Very good stuff and well worth beginning to practice. He got his beginnings by bringing together gangs and getting them to talk about what their needs are... turns out they all and we all have the same needs. Instead of yelling at each other and telling each other what they should do to make it right, they started talking about how they felt about what had gone on between them and asking how each other experienced the situation... they ended up realizing how they had similar feelings and goals and working together rather. His stuff is all over on video also. There is some stuff on the communication thread if you search for it on here.
Good luck. I love communication stuff! I'm actually excited for you because I remember how my own life was changed so much!