Don't get too upset.

You will get attacked for even just posting weather report online. That's how the cyber world is, people are tough behind the screen, and there will be no consequence to insult others. It's a given. Just take it easy, filter through useless stuff.

Speaking of your letter, personally, I don't see that is necessary. First, if you still want to stay, you should have and still can discuss with him in person, looking into his eyes. No need to put it down on paper. If you really want to put it down, and you don't want to stay any more, get his supervisor invovled as well. make sure you CC his boss, but you have to re-write the letter first. Don't get emotional, just cite all the facts, don't put in anything subjective, let others to draw the conclusion by themselves. In short, protect yourself, but be professional about it.