Why is this bothering me so much?

I was talking to my girlfriend about our respective future plans when I asked her if she ever thought about our future. She says she does sometimes but that she mostly thinks about how she doesn't want me to do what her ex did and change the college I want to go to to hers so that we could be together. We are graduating from high school next year and I know I would never be able to have a long distance relationship. I guess it's just the realization that there is no way we could last. I know I'm only in high school but when I think about our future I think about how I want to spend the rest of my life with her and her saying that made it hit me that what I want is never going to happen with her. I'm not really asking for anything, I just had to write it down somewhere I guess.