I think everyone hits plateaus like this every now and then. Getting out of them is what separates each scenario. I don't know exactly what to say simply because you'll find your own way to get out of it. But I can suggest a few things:
If you don't want to play, then don't play. It's important that you do what you want to do and what you like. Take a break for a few days/weeks/months. Maybe eventually you'll come across an old pair of sticks and start tapping and remember all the good things you got out of it. Remember, it sometimes takes one step down to go two steps up.
Don't let your relatives' opinions stand in your way. Sure, the attitudes of those around you can be effective of your opinion, but they are not the people that play. They are not you. You have your specific goals that you wish to achieve in your life.
I also want to add that Vanderbilt is a very good school, for both music and academics. I don't understand why your family wouldn't want you to go there. It's sad that your family doesn't support you in something you obviously love to do. They don't have to go to the
DCI shows (I bet you're not interested in everything your grandmother does) to show affection but what they are doing, but they don't seem to show any true emotion about it. Maybe they'll see what you currently see if you give them time, just like your mom does.
This sort of relates to my first suggestion, but it's not as extreme. Find a new form of percussion to do. The great part about percussion is that there are a myriad amount of options to follow through. You could play set, or world percussion, or even go to a few drum circles. The best part about drum circles is that they're much more relaxed than the structured form of marching.
Whatever you do, I'm sure it'll be the option that will truely make you feel good. Hope you find your true calling and make the best of it.