I am sorry things are hard.
My ex started using (heroin) when my daughter was a few months old. It was
devastating.
Alanon helped me a lot.
We (me and my daughter) left her dad almost a year ago. I am happy I stayed away.
I think he is clean now but I am not sure. My daughter enjoys spending time with him
(I never leave them alone for now) and he is more and more interested in her. She
feels loved. I don't have any plans to move back in with him.
If your ex has any desires to be a father to your child he will find a way to be there
for him without you having to put yourself in harm's way.
You cannot change him, you cannot save him and you do need to take care of yourself
and your child. I hope you have a bit of family/friends support around.
Do check out Alanon.