Banging my head against a brick wall

I can't be responsible for someone else's actions, can I? I am a mother and as a mother I will always feel partly responsible for what my son does even though he's not a child anymore but I can't tell him what to do. So why do I feel like it's my fault, the fact that he's fucked up again. He was doing so well. Everyone said so. The truth is he wasn't doing so well and I should have realised it. I knew it was too good to be true for him to stay drug free after years of being an addict. I should have insisted he had some sort of professional help. No one can be addicted to heroin for years and be expected to cope in the real world without proper help.
My sister rang to say they'd found prescriptions for methadone in his room. They weren't in his name. He obviously couldn't manage without and has been buying the stuff. My first thought was that he must be back on the heroin. So he's let everyone down and lied to everybody again. Why am I not surprised? He's now broken the trust of everyone who has bent over backwards to try and help him. My sister and her husband won't have him to stay at theirs now, no matter what he does....and I can't blame them. He's going to end up homeless again and I know what that means. If he's not using heroin now then it's only a matter of time before he does. If he's homeless he'll seek help from the very people he needs to stay away from, the dealers and other addicts.
I've managed to talk to him once. He's blaming everyone but himself. He swears he hasn't used heroin. He says he was under so much pressure to get off the methadone quickly, start work and prove himself that he felt he couldn't tell anyone or ask for help. He said he couldn't cope with his moods and methadone was the only relief. I feel sure that he has some sort of mood disorder, like me. Maybe bipolar, maybe not. I've tried before to get him assessed but it's impossible while he's still using. I'm sure he's self medicating but it's a vicious circle. How do they differentiate between mental health and substance abuse and how one thing affects the other? I do feel for him. I know what it's like to have everyone thinking you're fine and telling you how well you're doing, when really you're in bits. I can understand why he kept it all to himself but at the same time he should have been man enough to talk to people.
I haven't been able to contact him again. I have no idea if he is ok. I don't know what to do. I don't know if there is anything I can do. I feel like I'm banging my head against a brick wall. I feel helpless. I just want him to be alright.