Okay she sounds as though she is very lonely.
I don't agree you're being mean. That's nonsense. You're just struggling to cope with totally unreasonable demands.
this person is clearly uncomfortable doing her own thing, on her own at any given time.
There will be a reason for this - perhaps she is trying to forget something, she doesn't sound very happy.
That doesn't make it your problem to assuage her loneliness or absolute need to be with someone at all times.
I think oyu may need to offend her, if she is that close by. you could have an honest talk but she doesn't sound like she would be receptive to that. It may get rid of her though.
I'd be angry in your position because you're being used and ignored.
Try what someone else said, suggesting a single time and day to be free regularly. Maybe invent a new job you have to do from home. I think excuses are justified in this situation.
Please don't feel guilty. It's her, not you. Good luck!