As a husband who hasn't received such a letter but would love to, the overarching theme would be respect, appreciation and love. And that despite the criticism and anger, you really do feel love, appreciation and respect for him. And to forgive you when you PTSD has you in its grips. That you will, when you can, strive to show him you love him. And yes, thank him for all he does. I think it would also be okay to perhaps write a separate letter to ask him for things he's not doing that you need him/would like him to do.
PTSD is counterintuitive. Behavior that would typically be seen as being selfish or shortsighted or otherwise negative in a "normal" person are par for the course sometimes for PTSD sufferers (and my wife has Complex PTSD so it's even stranger). So, it is hard for supporters to know what is needed, even if we do read boards like this and books and go to marriage counseling.
Another thing is to see the movie "Enough Said." I just saw it last night, and though it isn't about PTSD, it is about how negativity towards a loved one can overshadow the love. And how not to make good or great the enemy of perfect. It is also just a nice, well acted, fairly funny movie! That may give you additional perspective.