If he's in early recovery he's not going to give you an in depth apology he's going to be workong on getting off the drugs along with the reasons why he took them etc etc. it also depends on the language in which you wrote the letter to him. For instance did you put you did this you that you hurt me n so Forth - or if you put 'it hurt when you did this that or other. Things get misinterpreted via letter.
All this aside what are you going to do
To make YOU feel better? Please stop trying to figure out his actions its impossible.
Take care
Evey xxx