Welcome HelloKitty. Glad you joined us. This SR forum is a great place for learning more about addiction and getting support. One of my favorite sayings to my H is "Actions, not words." (I've heard too many promises that were'nt kept to believe just words anymore.)
You'll see lots of different endings to our prison journeys here on this thread, and to find your path you'll need to look within yourself for the answers. I so get the struggling single mom part. That is ME too! I have 3 kids, and it seems there is something one of them needs all the time.... which requires $$$. I think you'll do just fine in writing the letter. Just tell him your honest feelings as you wrote above. If he is serious about his recovery, he'll work with you on this. For now, take time to take care of yourself. Do something nice for you and the baby. For so long living with addiction, we often forget how to relax and have fun. Keep reading and posting here. You'll find lots of encouragement & support.