I don't think the letter was too harsh. We don't speak that way to each other in person, so the letter was meant as a "wake up call". A kind of: this is how seriously you're hurting me, type of thing. Again, I've use NVC in the every day and I love it and think it works. But Pretty and I know each other very well. I've made a ton of concessions in this situation.
The point is moot, however. The night after I wrote it, I called him and told him that we needed space, and that I would send the email, but I never did. I needed to talk in person. The next day, we talked and he said he needed space to figure stuff out for himself. He's torn because he loves both of us. I need to be stabilized again and feel respected, so I suggested that we be exclusive for a while, until we work things out, and then open our relationship back up. He's unable to commit to leaving her (I really don't blame him for that, it's just that we're not getting anywhere in this V) and is overwhelmed with the intensely emotional repercussions of his actions. I feel terribly threatened that he could leave me for her. It's definitely a possibility. Perhaps a probability.
I'm prepared for a total break-up after this week, but who knows. I cannot stop crying. On the bright side, maybe I'll finally get some cleaning done...?