Disabled mother, painkillers and the will to get out of bed. What do I do?

I'm having a 15 year long problem. My mother has a plethora of health problems. So many, in fact, that she packed it in and called it a life at the age of 40. I was 19 at the time and trying to make my way through college. For my part, I'm angry that this happened just a year after I went to a very expensive school that I was encouraged to attend because mom had worked so hard to provide these incredible opportunities. I was on grants & loans and surely would have gone somewhere else had I known I would be on the hook for $100,000 in loans. See, she had a great job at the time and only got "sick" when she decided to leave her job and move closer to where I was in school. With no plan, no savings, no job, no apartment, and a dog in tow, she left her life and when she couldn't find a job or apartment, moved into my flat with my 2 college roommates. At that point, she broke down and hasn't recovered.
Cut to now, 15 years later. I've given her 1/3 of my income since then, found and moved her into numerous apartments (only to have her complain about and leave each one after ruining her relationship with her neighbors and landlords), and gotten her into a pain management clinic that helps her with the joint & muscle pain she's constantly in.
I'm angry things went down the way they did, but can't blame her for being in pain because obviously that's real. The problem is its been 15 years and she is only getting worse, not better.
She eats junk food, drinks coke & coffee all day, then complains she can't do anything every morning because of her "bathroom problems", which of course are because if God and not her bad eating. That leads to the early afternoon when she flips on the tv and sits in front of it chainsmoking cigarettes all day. At this point, she can't be bothered to get up out of bed, let alone get her daily gallon of milk or bottle of coke from the market or - God forbid - the grocery store.
And now, since the pain management clinic, which she claimed, no promised (!) would help her start doing things again, her #1 goal is to get messed up on pills until they run low so the pain subsides. She even thinks its the fun part of every month. And maybe for her it is. But for me, it's horrible. She doesn't know what she's talking about from moment to moment, which wouldn't be an issue Except that she doesn't stop talking from the minute i arrive until I'm walking down the stairs to leave. At that point in can clearly hear her continuing to talk to herself as though I'm still there as I'm walking to my car. That's annoying and I fear for her mental health, but it's the forgetfulness that is really a problem. Remember the chainsmoking? Well, she regularly drops cigarettes, now has burn holes in all her clothes and blankets/sheets, and laughs it off. Someday, one of 2 things will happen - She'll either burn the house down or her landlady will see what she's done to the carpet and kick her out.
That scares me enough, but what's worse is I can see the future. She'll show up at my door with bags saying she has nowhere to go and needs to stay for a "couple days" which, of course, becomes months, which becomes me moving out.
My youth is gone. My patience is gone. My sympathy is gone. But she is getting worse. What do I do?!?