For me, the most important piece of understanding 'detachment' was accepting that I was choosing, every day, to stay with my XABF, and to stay with all of the behaviors that came with him. Before that, I believed very firmly that how I reacted to his behavior would somehow have an effect on him, or upon the consequences of those behaviors. I believed I could save him, or me, or us, from the worst of those consequences.
When I began to let go of that notion, detachment was merely a matter of focusing more on me and my own responsibilities, interests, etc., and letting go of the emotional responses that came with my reaction to his behaviors. Getting angry at him simply didn't make him stop falling asleep on the couch with lit cigarettes. Getting upset didn't make him stop lying and hurting me and other people. Getting jealous didn't make him stop flirting with other women online. Instead of engaging with him on every little thing I perceived as his screw-ups and mistakes, I would instead leave the room. Read a book. Go out for a walk. Talk to friends. Write. Sleep. Dream. It was incredibly hard. My couch has a cigarette burn in it.
Eventually, not too long after I really started to detach, I made the choice NOT to stay with an active alcoholic with no intention of changing his own behavior.
I think it's important to note that detachment is not pretense. It's not a manipulation tactic to get the A to do something. It's more a way of being that moves your focus off of the A and onto yourself. You say, "it also means if he fails, our family fails." The hardest part of this whole thing is understanding that his success or failure is independent of anything you can do. That is the reality we must accept in order to make decisions about our futures.
You aren't dooming anyone. You are in a very tough spot, but you CAN come through this, and not in any prescribed manner, either. What works for one person is not necessarily what works for another. This is a long journey you're undertaking, and like the rest of us, you can only take it one step at a time.
Sending you strength and courage.