How desperate can a drug addict be?

What do you do when your only son is on the end of the phone and begging you for money? What do you say when he tells you he'd rather be dead than have to face up to the mess he's got himself into? What do you do when you know that if you give him money he'll probably use it to buy heroin? Everyone tells me the same. Don't give him money. If I do I'm just enabling his addiction. It's not that easy when someone you love is crying and begging for help. He promised me that he'd hand himself in to the police but needed money to buy Subutex (a prescription drug used to treat opiate dependency) because he felt so ill and couldn't face doing it without. 
What do you do when he sends you a text like this? ...... 
 "I'm your son. If I had a son or daughter I'd do anything for them, drug addict or not. Why? Because I know what it's like. I'm not stupid, I know right from wrong. I know you think you're prolonging me from sorting myself out by giving me money and you're right but I'm ready to do it now. Everything bad that's happened in my life is my own fault because of drugs. Don't ever blame yourself. I just want your help one last time. I need £40. Love you x" 
I gave him the money. I don't know if I was right or wrong. I just pray that tomorrow he'll do what he said and hand himself in. 
Of course he didn't hand himself in. I'm such an idiot. It just goes on and on. I should cut all ties and let him get on with it. I can't do it. When you have a child, that child becomes your world. As a mother you would do anything to protect them. It doesn't stop when they become an adult. It's a life long commitment. The problem is I haven't really been helping him. The thousands of pounds he's had from me over the years hasn't helped him at all. I've just been enabling him to feed his addiction. I hate myself for that. I do feel guilty because I am guilty.