I agree with WSarabians and the others who say that for the moment, you must concentrate on yourself, and building a future for yourself, since the adults in your life seem unable to help you in the way that I would expect a parent to do.
I think it best that you let her go to someone else. I would start right now trying to find her a new home. Look into a rescue. Or a school, or even a breeder, if she has papers. Find someone who will take her for free or cheap. Then, leave a proposal in written form for your father, with the contact #'s and make it easy for him to unload himself of this burden.
The only worry I would have is that he would sell her for meat. Do you think he would do that?
Got get the job at the barn. Help this lady and start helping others with THEIR horses. You'll get the satisfaction of being in the equine world, while not having the financial burdens.
This is a big turning point for you. Time to grow up. Kind of sudden and harsh, and not fair, but it's here, nonetheless.
That's what I'm afraid of....her ending up at a slaughterhouse. Its so likely for her...with the lameness situation, and so hard to find a home. I have absolutly no problem letting her go, I mean it would be hard..but I could deal with it. I just want to find a good home for her.