didnt get a chance to read through the whole chat here, only the first part.
have you thought that maybe you kept the letter so by chance dh could find it? not to hurt him but just so it would open up a channel of communication between you guys? it seems like you've been thinking these thoughts for a long time - on your own. you probably have really been wanting to be open about it with him. sometimes if you are open from the start when things start to bother you, you talk and realize its more in your mind then for real and things quickly go 'back to normal'. the more you think about it on your own in your own head, the more it becomes bigger, and out of proportion from reality, making it much harder to ammend.
btw, it cant be right that things were going downhill all because of your problems. not having the openness to share and get help form your dh is part of his problem too. maybe after your csection you didnt give yourself enought time and space to recomver mentally and spiritually. that could have been a time when you needed dhs understanding more than ever. and the more you kept it to yourself / he didn't understand/support your trauma,the further away you got from him.
i hope your doing better today. usually the day after things look a little more optimistic.
hugs