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First... I am sorry for what you are going through, it's a rough state to be in. I would like to give my 3 cents on the letter to your son, because that seems to have the highest potential for lasting consequences. Whether you agree with m or not, take it or leave it, up to you.

The first part Clodfobble pretty much already said it:

"You will never find someone who loves you as much as I do" says as much about how he is borderline unlovable as it does about how good you are at loving him. It's the kind of phrase that is never intended to hurt, but pretty much always does.

That strikes me with much of the same meaning. I get what you are trying to say, in that you feel so much love for him you don't think it can ever be matched, but it has the unintended consequence of devaluing him in a kind of possessive way - telling him he can only ever get it from you - which actually makes a bit of sense, with your husband just leaving and now him seeming to go the same route, it's natural you might on some level fear loosing him.

The second one - and that's a big one - is blaming him for "choosing to feel angry".... were you choosing to feel that "dark cloud" during your relationship? If things worked that way everyone would be happy all the time. Some people do have some self discipline regarding what they allow themselves to think of and thus what feelings get built up inside, but its a relatively rare characteristic (And can be as destructive as it is helpful, so if your son doesn't have it you are probably better off).

That combines with the 3rd one... I don't know how it is with your son, but I know for myself, if I was angry at someone, that kind of letter would never help me forgive them. In his position I might put the anger aside for the benefit of the relationship with the brother or even the family as a whole, but it would actually make it harder to forgive whatever it was that I might be angry about, which would mean that the best case scenario in reaction to that letter would be stomaching it until it surfaces again (Worst case scenario - incite him to give up completely).
I do know there are some universal elements to this - we know from the extreme case of trauma victims, which I am not saying he is but I believe this is true for lesser offenses as well, that the healing starts when the perpetrator acknowledges and admits to what they have done, acknowledges how they've impacted the victim and expresses regret. Which makes perfect sense, as long as the other person doesn't show they believe they shouldn't have done something then there's no good reason to put aside the fear they wouldn't do it again.
Even if you did nothing wrong and all is well, his anger suggests that's now how he experiences things. Saying how you've always done the best you could have with what you've known at the time puts you in a very good light, and might be how you genuinely feel, but from his perspective it would more likely then not serve as denial for whatever pain is underlining the anger.
Until you swivel around the issues and find the perspective from which his anger is legitimate, his own perspective, even if it makes you feel shitty as hell while you do it, and it almost certainly will, you won't be able to see where he's coming from, why he thinks whatever he does, and how you can make sure he won't feel like that in the future and help him heal the pain he IS experiencing.
...And this is the important bolding - it might not be your fault, it might be disproportional (At that age it's almost certainly is), it might even be derived of complete misunderstandings or an outright bullshit set of values, but he is experiencing it, or he wouldn't be acting this way. It is real for him.