Addictions stem from depression. You stated you fell into depression. Within relationships partners often mirror each other. It's possible he was deep in a depressive state, maybe functioning depression, and hiding from reality by using you to be the go-between. He didn't want a caretaker, an equal relationship, he wanted an enabler. He needs help on some level you can't do. He needs professional help perhaps and to do the work on himself. This isn't your responsibility and moreover it's impossible for you to do for him. We all have our own journey to contend with. Depression is a spiritual issue, an emotional one, a solo experience one needs to go through. Sometimes when others try to help, all they do stave off the experience and make it drag out. People need to learn to pick themselves up sometimes. Think rock bottom. With addicts, sometimes that is the only way they pick themselves up.
Now, for the you aspect. Selfishness is needed here. You focused so much on him, caretaking him, you forgot about you. Fell into depression yourself. That is important. That is your needs screaming at you to be met. It's time to stop caretaking everyone else and caretake yourself. You are amazing at doing for a little, if you gave yourself a fraction of what you gave him, you would be so much better. Try focusing on you a day out of the week, only you. Or 3 hours a day each day devote to caretaking your needs. Remember you are in a mental health crisis too. You are depressed too. Try therapy, a new hobby, meditation, make a new friend, change your routine, sleep. Do something that benefits you and no one else.
I don't know if this needs to be said or if it's my place, but I think your relationship with him is over. Maybe it's time to grieve. Remember you did everything you could, but it wasn't enough because he needing to be giving to himself. It can't just be water down a well, a relationship is about an energy exchange. He should have been pouring all that love and care back into you as you did for him. The fact he couldn't means he needs to learn how to fill his own cup, be his own source. You know how to do that, you've been a source of love and devotion for him for 6 years now. Stop. See how fast your cup fills when it's just about you. Be selfish, take care of yourself. You are the most important person in your life.
I also want to reiterate what others said. You are an amazing Mommy for a little. Every little on this site would give their eye teeth for what you gave him. Don't let this break your trust in yourself. You know your own worth. Don't forget it.