That's the best advice you can get -- take care of yourself. Put him out of your mind, no matter how hard it may be, and don't humor him with this "it's all your fault" or "I'm trying to get better" crap. Convince yourself that he is lost, because there is literally not a damn thing you can do to fix him, change him, encourage him, or make him quit using. It has to come from him. I hope it does, but either way you won't be there to see it. Honestly, at this point, there is so much resentment and pain embedded in your relationship that it can never go back to the way it was in the early days. Your marriage is broken because you will never trust him again, and you deserve a partner that you know will always have your back.
I know first-hand how hard it can be to cut off someone that you still love with all your heart. Someone who is a good person, sweet, caring, and kind, but crippled by an addiction that turns them into a monster. But keeping a hardcore heroin addict in your day-to-day life is only going to bring negativity, pain, suffering, and anxiety, and you need to surround yourself and your child with things that are positive.
You can't save this man, and he has nothing positive to offer you. You can save yourself, and you should.