My take is that the letter is his way of keeping you hooked to your drug- that would be him.
If I recall, you spent 7 years in a relationship with another addict and here you are, once again, doing the codependent cha-cha. Sounds like a pattern to me.
Regardless of what you do or not, his addiction is going to get worse because it's progressive. Given he's " slammin pills", jumping off into heroin is likely just around the bend.
Protect your SSN, valuables and credit cards. Double up on the ole birth control because a baby does not snap anyone out of addiction.
Continue with therapy cause there are reasons why you are choosing broken men. Back when, the more I fixated on my daughter and her problems the less I focused on myself and my problems, including my need to rescue and fix her.