Making the decision that you cant live with his drug behaviors is a big step & Im sure there were lots of reasons why you feel this way. Maybe try doing some journaling, or making a list of some of these things as a reminder. I would also suggest making a list of the things you do want to bring into your life. In other words, try to stay focused on the positive, envision what you want your life to be like, EVEN if he does not get clean and make the journey with you.
I think it was great that you wrote him a letter and reaffirmed that you want him in your life, but you need him to be clean in order for that to happen.
When my husband was actively using opiates, he got to staying out late, he would lie, we would argue. He was never violent or anything, but I knew this was not the type of relationship I wanted, I was never going to be happy putting up with that behavior, and our marriage probably would have deteriorated quickly. So he finally moved out and it was also with the understanding that when he was done, then he could come home and we would see where we stood. It took him a year to be done, and then he asked for help.
I had to go about my life as best as possible during that year, and although I stumbled often, I kept heading forward. I tried to design my life so that I could keep going on my own, or if what I desired came true, then he would meet me along the way and it would still be the right path.
My husband and I didnt have kids at that time. So I dont know how you will handle that situation. He probably still has rights to see his child if he is somewhat stable. Take it day by day.