I can definitely relate to your frustration, JSM. Addicts often act as if they are entitled to use because they are soooo special, and that attitude drives their families and friends up a wall. When they are actively using, they do idiotic things no sane person would ever do and rationalize ("I know what I'm doing!" ...ergh!) "You just don't understand" sounds like the kinds of things my addict would say as she maxed out another credit card, and that drove me bats*** crazy.
But the truth is there are lots of things I'll never truly understand about my addict--and about all of the people in my life, from my parents to my neighbors and friends. That doesn't mean a loving/trusting relationship is impossible. What's important to me is not understanding--at least in the sense of knowing all about why she did this or that stupid thing--so much as compassion. Both AA and Naranon help me to enlarge my spiritual life, and that, to me, means enlarging my ability to forgive, and to *let go* when necessary--of my desire to control my addict, of my need to hear her apologize repeatedly, and even of my need to believe that I understand.
I'll never understand why drugs took over her entire life, and she'll never understand why I can't touch alcohol. But we can, and do, forgive.
And I'm not saying this so I can sound like some enlightened being--it's hard work, and I often don't do it well. But...one day at a time, as they say!