Hi my first post as well, I also have an adult son from a divorce some 20 years ago.
I often feel the same that he is using drugs and alcohol to mask the pain of the split. He often reminds me it was our fault, that trauma in his childhood has caused this. It certainly escalated when he went to university and I had to get him home in year 2. But eventually had to ask him to leave our home.
Over the years like you I feel I have tried everything, paying for rehab, being financially drained, recently again paying a deposit so he is not made homeless but he lies again and again and is unable to work, so will be evicted I am sure soon. It’s relentless. I do really work at my own mental health to try and remain working and able to manage.
but as you quite rightly say until he makes the decisions for him self nothing changes.
although I know this, I get sucked in time and time again. As a mother this is so bloody hard. Sending hope and solidarity x x