I would put the foal down.
Not only is there the possibility of some other, more serious, health issues going on, but making sure she has proper care and training is going to be very tough and, likely, expensive. It will require you to completely renovate your property, getting rid of all wire and anything sharp that might possibly snag her at any point, putting a wide swatch of gravel or some other alternative footing around every single fence line, making sure she has an appropriate pasture companion (not all horses make good seeing-eye horses), etc.
Training her to be ridden creates a whole new set of problems. What if she doesn't have that innate "born broke" temperament? What if she goes through a phase of bucking or trying to bolt? Unlike a sighted horse, she can't avoid obstacles and is, therefore, a tremendous risk not only to herself, but to her rider. It's one thing to own and continue to use an older horse who's sight is failing. They already know all the cues, they know to trust the human and obey. A young horse doesn't know any of that.
Another thing to think of, even though Corporal may have made the point a little more gruffly than necessary, she's right. What are the odds of finding this filly a home in the event that you are no longer able to keep her? Even if you did somehow get her broke, who in the world would want to buy her? I wouldn't. I wouldn't even take her for free because of all the inherent costs and risks associated with owning a blind horse. I'm not trying to sound callous, but I feel it's better not to sugarcoat my opinion on the matter either.
If she were at my house, I would spoil her completely rotten for a few days and then have her put down. Then, I would find another, useful, horse in need of a good home and give them one.