You are wise to make this decision @countrygirl
. It's often a dream to own a horse, but sometimes, it's best to stick to lessons for a while longer. And in this case, you didn't really "own" it anyway, and didn't really want it! Tell them you're done, they can sell the horse, whatever, and you appreciate the opportunity, but have decided you need more lessons (if they insist, tell them you can't afford lessons and to continue paying for this horse). Hopefully they will understand, after that lunging incident. But if they don't, it's not your problem. This is not the horse for you - and you are smart enough to know it even though they aren't.
As for the vaccines, 150$ does seem like more than your share (it sounds like it should be 100$), but that's really not excessive for annual vaccines. Ask they why you owe 150$ if the total bill for 10 horses is 800$. Or did some of the horses not get vaccinated? Did some have their teeth done as well (I get my horses' teeth checked and filed at the same time they get their annual vaccines)? You may just have to fork over the 150$ since you previously agreed to care for this horse, but tell them it's the last time, and that you're done with this horse.
Best of luck to you! Enjoy riding a good horse for a while. And good for you, for recognizing this situation was unacceptable.