My girlfriend has no energy and severely depressed and is probably mentally ill -- how do I get her to a doctor or persuade her to take her meds?

I am not financially solvent enough to take her in, but she once did better than this and had a life.

She blames all of her problems on a past sinus surgery. I do not doubt that she in pain from this surgery. But it is neuropathic pain.
She won’t even take pain meds, saying it only fixes symptoms and not the cause. She says she needs reconstructive surgery but one of the best specialists available said no Her problem is not severe enough.

This woman, age 41, is extremely depressed (denies it—just says she is “in pain”), lonely, scared, very isolated, and living with her sister who puts up with her but nothing more (and collects rent, of course). My girlfriend has not worked in years— I rescued her from homelessness by exhausting my resources and reconnecting her with her sister. She never leaves the house and seldom her room. She is afraid she is dying. She wants me there ALL THE TIME and makes me feel guilty about leaving her side, no matter how much I do.

I want her to enjoy life again and be her own person.
What can I do?

In short, she is miserable but afraid or too lethargic to go to a doctor.