17 year old son taking drugs

My nearly 18 year old son has a real problem with drugs. Since he was 12 he has been smoking weed nearly every day. He’s always been in trouble and caused trouble and was thrown out a few schools.his father is an absolute waste of time, we separated when my son was 4, and he is in and out of his life. He makes false promises to him and let’s him down constantly. I feel the drug use is to block out his feelings of neglect from his father. Anyway, over the last year the drug use has got worse. It’s now ecstasy nearly every day, drone, cocaine and weed. Three weeks ago he was at an all time low and I took him to a local drug centre who said they would see him weekly and help him. Unfortunately he never went back and the drug taking has got worse. 5 days ago I found messages on his phone that showed that he was selling pills to others. He’s lost his part time job as he went in out his face on drugs. After finding the messages I told him I could no longer have him in the family home. My husband & I have tried to do everything to give him, his sister and my two children with my new husband a good life. We got him a job, he choose his hours, we drove him to work. Gave him money for college, he had great holidays every year. For fear that his sisters may find drugs and because I feel I can’t condone it I have had to let him go and face the real world. I explained to him that if he wants to do adult things he has to accept there is adult consequences. I also keep sending him messages telling him that I don’t want to go on about the drugs but we’ll always be here to support and help him in any way and love him with all our hearts. Knowing he is couch surfing etc is killing me inside. I feel like a complete failure as a mother! I’m crying every day that maybe if I’d done something differently he wouldn’t be where he is today. Does this pain ever go away?