Hi LonelyStar: Glad you came back here to vent your frustrations. There are many of us who have gone the route you have with exactly the same story ending.
If you find yourself losing your resolve then come back here and start reading some stuff - read your own stuff. Read the stuff of those recovering from substances. I find that helps me.
I have seen the addict referred to as King Baby. I thought that was pretty accurate. Here they are running through life like a Mack truck in a china shop, and here we are jumping at their every request or running after them trying to keep them from breaking anything (including themselves). They just look our way, and we are right there ready to grant them every request (heck, they don't even need to ask - we can sense what is going to happen next and try to ease their pain). In other words, they say "jump," and we say, "how high?"
But it took me a while to have the courage to say "no" to my son and stick to it.
Glad you're here. It takes courage to change. Your Robert Frost line really fits what you are going through right now.