OP -- I join the chorus of people saying it's ok to find her a new home but I wanted to add one more thing. As of today, whether you sell her or keep her as a pasture puff, The Struggle is OVER. You are no longer required to go out there and attempt things that make you feel awful and afraid in hopes of someday being able to ride without fear. (Really with skiafoxmorgan on this -- I call BS! It's been FIVE years of weekly lessons for Pete's sake!) Please believe me when I say you MORE than gave this a fair shot.
Take a deep breath and from today, you will only have successes with Frosty. That might mean all you do is groom her, maybe ground walk her around a little. Maybe not even that. Whatever you can do that is a positive interaction and something you know you can do, that's what you do. And you don't feel guilty, and you don't feel ashamed, and you don't beat yourself up. Accept Frosty for who she is right now -- the horse you love but can't ride -- and figure out what that new relationship looks like. Maybe Frosty really likes a good grooming. Maybe she enjoys lunge work a little. Perhaps she'd enjoy learning a simple trick or two for some yummy treats.
Give yourself permission to just ENJOY it again, in whatever limited format that takes. I think, especially if you do sell her, you will greatly value having made the final phase of your relationship one of simple pleasure ,which, again, may mean nothing more than pats and treats. And that's really, honestly, perfectly ok.