This whole thread has made me very sad for the poor horse. I hope you view it as a life lesson. If you have no control, perhaps in the future, when you DO have control, you will not allow any animal to suffer like this.
Please put yourself in her shoes/hooves. How would you like to get the same treatment she was given? It is not rescuing an animal when they must endure this kind of treatment. This is precisely the sort of thing they need rescued FROM. Starving isn't the only "rescue" situation out there.
If, next year when you reach adulthood (presuming based on U.S. law) you decide to own horses and be responsible for another life besides your own; here is some advice to help you when these situations occur. First you need a relationship with a good veterinarian. You get this by using their services and paying what you owe. That way, when something serious comes up that is expensive, they know you will pay them so will allow you to make payments.
It has already been suggested and it is a great idea to have a separate bank account JUST for the care of the animals you choose to be responsible for.
In addition to the above, I personally have a credit card set aside just for my animals in case they need vet care and I am short on cash. I had a mare colic very badly a couple of years ago and required nearly a whole week in ICU at our vet hospital. I did not have the cash to pay but I did have the credit card so my mare did not have to be put down.
Lastly, if you can't provide care IF and WHEN it is needed when it is YOUR decision and responsibility; PLEASE do not take in any animals until you can. It is not rescuing if you can't care for them. I hope the mare you posted about does not have to suffer very much longer. It really is cruel. I agree with loosie who said :
So the way I see it, you have a few choices; keep on doing what you're doing & most likely get(& give her) what you've been getting, or worse. Get the horse into an equine hospital for adequate care & cross your fingers. Find her a home where the owner is willing to do that for her. Or put her out of her misery.