Since you've been dealing with this for 20 years I do not feel at all qualified to give you advice but I wanted to send along my support. I wouldn't wish the pain of being a mother of an addict on my worst enemy.
Anger is something I'm just not capable of.. I can find a way to justify the actions of anyone. Am just recently starting to see that this is not serving me well. Since I know I can't rely on anger to make me let go I've found another way. Anytime I have the urge to "help" my son I envision that the money I'm spending is going directly to buying him drugs (because that is really what it's doing). Any dollar I spend for his rent is another dollar he can spend on drugs. If I buy him a bag of groceries I might as well just buy him a couple bags of drugs because I just freed up his money to do just that.
Right now my son is in jail so I have a bit of reprieve (as horrible as that may sound) from having to toe the line and deal with the consequences (my own guilt) of telling him "NO". I hope that when he gets out I can continue to be strong -- and I hope that you can find the strength to do what you know is really best for you son -- letting him go.
((hugs))