Hmm, Dustin that sounds really tough.
Yet, these thoughts ARE irrational because she KEEPS telling you that she wont leave and IF she does go to Texas, it wont be for a very long time. I know you want her there forever, but just because she leaves (which I doubt she will, definitely not for a LONG time) it does not mean that you cannot still speak to her by phone or email.
Secondly, yes i can see that you would be very close, BUT you are only 17 (i know it is annoying when people say you are young, but i am younger than you) and so friends WILL change. you will be graduating soon and may find yourself getting closer to other people who mean more to you.
Lastly, she keeps reassuring you, so don't let an irrational fear ruin your appointments.

Just try and keep calm, okay? Take care my dear xxxx erica