I am so sorry. This sounds horribly painful. Our daughter's situation is different, yet the same. She's been diagnosis'd with ADHD and Bipolar Disorder. There have been other theories re: diagnosis as well and she has some medical issues. Knock on wood, she is not a drug user. Mood swings and impulsivity are her major dilemmas.
I have some ideas..but honestly, I am not sure if there are any absolute right and wrong answers here. Just ideas....
The family is always deeply affected when someone in it is ill. We've had many difficult years. It did get easier when she moved out of the home. However, I think her ups and downs may have worsened.
My husband and I went to a few meetings of Families Anonymous and found the group helpful. Ironically, the parents there all have one thing in common...their kids are substance users and their use caused difficulties and confusion in the home. Our situation is a bit different, yet we still received benefit. It is hard not to "buy" into the chaos, since these are our kids and we want so much to help them. All totally confusing since they are unwell (when there is a clear cut diagnosis, for example).
But even those who are ill have some degree of control. AND even those who are unwell need to make a decision to do their best to get better. To see a doctor or p-doctor or t-doctor for example.
I'm still processing it a bit myself...all soooo hard. I have found comfort and wisdom in setting up boundaries. Many here like a book called Boundaries by an author with- the last name of Cloud. Sue's link at the bottom of her posts on detachment is very good. Groups like FA or Alanon can be wonderful!!!! Please consider checking them out if you have not done so already.
Especially since your adult child is younger than 21, I would (for now) consider paying for the apartment and also consider paying for him to get good medical care...especially the services of a psychiatrist. In fact, especially if he is willing to put forth effort, I would be willing to help out with medical care for some time.
Of course, I would encourage him to go to AA (or NA) perhaps print out nearby locations from your computer 'cause chances are he either doesn't have on-line access or doesn't have it regularly.
However, you can lead a horse to water....can NOT make him drink.
That's about it.....you can't torture yourself and let your personal needs go unfulfilled because of this. You did not cause this. You can't live your life in fear or anguish. Never let him or anyone abuse you physically or verbally. If he calls you on the phone and is verbally abusive, hang up. If he comes by your place and is rough with- you...call the police.
At age 21, you probably should seriously consider pulling back even further, but for me personally, I would be willing to do a few extra things right now (on a limited basis, at a certain distance and with boundaries in mind).
At age 21, think about ending support (with the possible exception of some medical needs). Think about giving him some numbers of social service agencies and letting him know that you will help him with- his medical needs if you are able and he is cooperative.
Since he might have bipolar diagnosis, the services of a psychiatrist would be important in terms of better health. Medication (if he is willing to take it) might be helpful. But, don't help him out medically unless he is cooperative and you are able to do this financially, etc. And protect yourself, by limiting your involvement and absolutely, positively "detach."
If he has been making sincere efforts to stay out of trouble with the law and is seeing a physician and is polite to you, but his mental disabilities get in the way of him being able to be employed, I would inquire about social security disability and provide some assistance in helping him complete the forms, etc. But I would not help him, if he is not trying to get his act together and/or is unkind to you. I like to go the extra mile in terms of medical needs, but one must draw the line at some point. These "kids" have to feel the consequences for their inappropriate behaviors and sometimes/many times....those consequences are steep.
Regardless, I believe you would want to be rapidly moving toward detachment and by the time your son reaches the age of 21, I believe you would benefit by being immersed in the concept. You have your own life to lead and its very important that you do this very thing.
It's hard to believe right now, but there is a good world out there..even with- this turmoil going on. You owe it to yourself and your other teenager...to make a choice to look at this differently. Also to nurture yourself. Let go of the pain and reach for happier thoughts. Please go to Suz's link...read the entire thing; the "Letting Go" poem at the bottom is exceptionally good!
This detachment thing might be hard, but it is well worth the effort. Sending you good thoughts on your behalf and lots of hugs.
p.s. Witz mentioned the Serenity Prayer. It is very comforting for me...such wisdom and truth. A powerful prayer indeed!