I have a 15 year old dog whom I got as a rescue when she was about 7 or 8 years old (all estimates). A day ago the botto

I have a 15 year old dog whom I got as a rescue when she was about 7 or 8 years old (all estimates). A day ago the bottom portion of her third eyelid started showing more, and is covering half her eye. I tried a few of the online home remedies like warm water, wetting drops, and gentle massage, and it actually fixed itself for a bit, but now it's back.
What can I do, short of surgery?
Otherwise her other eye is fine, there's no puss or water, and she's in good health.