You'd have to give up your job, your horse, your friends. How does she know you'd be able to find work? If you moved to a more expensive area and couldn't find a job, you'd be in trouble. And if you did find a job, you'd still be scraping by. That's no life.
It sounds like you've got it made right where you're at. I wouldn't give it up. Your boyfriend's mom doesn't have it so bad. She gets to see her BF twice a month, which is more than a lot of people get to see their loved ones. She has two people that are taking care of her 24/7. I don't think she has much room to complain. Two hours isn't that far away. And it's not like they're married. If things don't work out between them, you would have moved for nothing.
I say let her stay with another family member if she wants to go so bad.