I don’t think you “owe” them an explanation. But since it is so hard for you to distance, I’m guessing that you have felt responsible for trying to fix their situation. It may be that you are struggling with letting go of that responsibility.
As you have discovered through hard experience, all “the support in the world” won’t make them change without their willing participation. They are mentally ill and addicted. You can’t be more invested in their recovery than they are without being disappointed and hurt.
You might feel easier about your decision if you framed it as a break, not an abandonment. Simply tell them that with all you’ve been through in the past year, you don’t have anything left emotionally to offer. If you are up to it, you can periodically let them know you love them and that your door is open when and if they take their recovery seriously. It’s not something to discuss or argue about. There’s no need to go into further explanation than that. I’m just guessing that in your case, saying something might feel better than cutting them off.
I wish you well.