Soul, I think the key to detaching is in simply not feeling responsible for it, or responsible for fixing it.
Perhaps you can better apply this in a hypothetical relationship. For example, imagine that your neighbor whom you really like has a drug problem. She needs a fix and asks you to give her some money. While you might care for her and sympathize with her problem, perhaps even offer some constructive advice that she should probably seek help to break the addiction, you don't feel compelled to give her money (at least you shouldn't.) You might offer to drive her home, might offer to look up the number of the rehab place for her, but the rest of it is up to her. You CAN'T fix it for her, even if you handed her the money.
This might not be a good example because it might not be personal enough for you. Is there someone else in your life that you can try to apply this to, just to mull it over in your head? I think if you try to apply it right off to something as up-close and personal as your M, it might be too much to bite off. Or at minimum, pick a tiny aspect of your H's actions that you can separate.
It's sort of the idea behind not being so available for your H when he calls or texts. You don't need to be rude and mean, but you don't need to hand-hold him through this either.
