How do you tell if mother has Dementia or general age-related decline?

Please forgive the long post. It's difficult to explain what's happening with my mother without telling something of her history.

My mother will turn 80 next month. I didn't know/understand this until about a year ago when I came across the definition for Borderline Personality Disorder, but she falls almost exactly into the Waif category. She has always been a "helpless" yet manipulative person, threatening suicide with classic signs of depression, heavy wine drinker, has never able to hold down a job, is stark-raving terrified of abandonment and is the least mentally-focused person I've ever known.

Twenty years ago a friend commented that my mother's way of communication was very "affected" and that she had a "charmingly childlike personality." He was right. I sometimes think of her as a teen who's trying to pretend she's an adult. Another thing that has always frustrated me is that she is unable to take in anything she doesn't want to hear (i.e. the need to go to a doctor when necessary, see someone for depression, a frank discussion of how her behavior effects me). Instead, she'll cry, say she's a terrible person who should just die, and then moments later it's like the conversation never happened.

She used to go through mood swings and crises from hour to hour or day to day. Nothing could ever be in the middle; it was either good or bad. She adopts extreme behaviors, solutions, diets, values, etc.

Beginning eight years ago, until last year, she used to insist she go everywhere I went. I couldn't go anywhere without her coming along. She told me that if I died in a car accident, she wanted to go at the same time. She wouldn't know how to survive or pay bills, and would put our pets to sleep and then kill herself if something happened to me.

I've worked at home for the last three years, but before that, she would freak out if I didn't come straight home from my job. If I stopped at the store to pick up some groceries, she'd be on the phone asking why I wasn't home yet, in a sweetsie, yet panicked manner.

That has calmed since I quit my job to work at home. If she has a problem or a complaint, I've learned to fix it as best and quickly as possible. I now make just enough to get the bills get paid, but couldn't support us both separately, so she knows I can't leave.

Okay, so that's the history, now the problem.

A year ago my mother took a hard fall and ended up in bed for months. She refused to go to the doctor, even though the pain made it impossible for her to get up or do everyday tasks. She healed enough to be able to walk around the house and occasionally go with me in the car, but she now is confined largely to home. She hasn't taken a both or shower since the accident and basins instead, because she can't lift her leg over the side of the tub.

She has diagnosed herself as having piriformis syndrome, basically sciatica to the 10th power. I've begged her to go to the doctor. She won't. She doesn't want a doctor to see her "ugly body naked." She doesn't want to end up hooked on painkillers. She doesn't want to have an MRI, other tests, or surgery, because she's read online that surgery is rarely successful and there's a good chance of paralysis.

Instead, she self-medicates with wine. I'll admit, I'm the enabler here, but without it the complaints and dramatic cries of pain would be multiple times worse than they are, and though this probably makes me a bad person, there is only so much I can handle. She needs to grab onto everything in order to move around or even keep from falling over. She tells me it's the sciatica that requires her to hold onto everything, but I honestly can't tell if it's that, or she's simply drunk. In the rare when she acts more clearly, she still walks with difficulty, but doesn't need handhold after handhold to propel her forward.

Lately, I've noticed that the "mental" component of her habitual helplessness is getting more pronounced. She repeats things often. She can't follow simple logical trains of thought. Her responses during conversations sometimes don't match what is being discussed.

My question is, how do I tell if her cognition problems are part of her way of interacting with the world, due to intoxication, normal aging, or the early onset of dementia?

Many of the indicators people list as signs of dementia are already a part of her and always have been. My mother has never been able to remember her own phone number or address (I've supplied them on forms for her since I was a teen). She makes up cutesy names for things she can't remember, but she's always done that. She has difficulty learning new things, even very simple things, but she's always been this way.

If anyone has suggestions, I'd appreciate it. I don't know if I can get her to go to a doctor, and she's definitely not so far gone that she's putting her toothbrush in the refrigerator, or forgetting who I am. She simply appears less and less "able" mentally.