If people are being rude to you they may not be nice to your horse either. They are nice to your parents as they know where the money comes from. If it was me, I would be already interviewing other boarding places.
Another bad sign is you said a lot of people are leaving your current place. If people are leaving the problem isn't going away. If your worried about leaving the mare, ask your parents and the mares owners about purchasing her.
Lastly, whats more important, your friends or your horse? This one should be simple. Just remember, you already own one horse that needs cared for and should be your primary responsibility. You can only do what you are able. Good luck and I hope it works out for both horses and you.