Dear Tired,
You have come to the right place. Sometimes all we have the energy to do is cry for "help" but that's ok. This forum can, to the degree you work it, provide you with courage, strength and hope.
Our difficult children can keep us in the "FOG" (fear, obligation and guilt) and that is a very haunting place to be. We don't have all the answers but in my experience since sharing here, it is helpful to learn the "stories" of others and know that we can survive this but it is not easy and it is a day to day process.
Your son is younger than my two sons. I have a 26 yr. old who has been living in his car and homeless for about the last 3 yrs. He nearly sucked the life out of me. I provided for him financially until almost everything I had was depleted. I actually have a 31 yr. old who I had to distance myself from over the last 8 months for similar issues.
Younger son, since 12-13 was smoking pot. He has ADHD and PTSD and at one point smoked pot like it was his job in our house, verbal abuse and physical abuse. He's broken my ex-husband's nose, punched my older son's front tooth out along with numerous holes in walls, doors etc., too many to count. At the time, my husband and I were married and we tried for years to put him on the right path. My ex-husband (of 30yrs.) was an alcoholic too so the mix of everything was just too much.
It is a difficult process, but from my own experience the only way I got better was by changing myself. I sought therapy, read good books pertaining to my issues, prayed and prayed and prayed. I also belong to Al anon and this has helped me tremendously. It has taught me to stop trying to rescue everyone from their problems. We didn't cause it, we can't cure it and we surely can't control it. It's when I rely on my own self to fix the problem, my life becomes unmanageable. For me this has been something only God can fix. Turn your son over to Him (if you believe) every time a thought comes into your mind about him. It doesn't mean that even though you rely on God that all your problems will go away but I have found that in the long-interim of praying for my sons, that God has changed me.
Stay strong, keep posting and let some of the chunks of the heavy burden you carry stay here on this forum. Once you expose a problem the magnitude of it does lessen a bit.
Prayers to you and your family.