I would talk to your Medical Director and come up with an ED care plan, you do not need a PCP for that. On some of ours over the years we would have a plan in place to only treat objective findings (i.e. withdrawal s/s). We would have a plan to NOT feed them, to NOT give them a treatment room. If they were SI the plan would be to put them in a hold room with nothing but a mattress. Some of this may seem extreme but the only way to decrease the abuse is to take away what the reason they are coming to your department.
As far as for you, please be careful to not let your frustration show in your care, and do everything you can to not bring it home. I feel sorry for someone like the person you describe and feel that it must be sad to be killing yourself just to escape whatever he/she is trying to escape. Many cases came to mind in my own experience that are nearly identical to what you describe, the last I can remember died in an ally after passing out drunk in the winter. She was 26 and in our department at least twice a week, never wanting help just angry that someone called the cops to land her in our department.
Just keep swimming and really appreciate those cases where you can make a difference. Drug and alcohol addiction is an illness in my opinion and your patient is afflicted. If you think about it in that sense it might be a little easier to handle.