@Role up,
Be your usual kind, supportive self.
I get the feeling she's seeing the future and it contains school or work but, possibly, not you. Sorry to say that, but you are both changing. Neither of you are who you were 3 years ago. No one is, but that is particularly true of teenagers.
And it does not have to be another guy causing her to act and feel this way (if I am right;
perhaps I'm totally off base here). The truth is, the vast, vast majority of relationships don't work out, and the percentage is considerably higher for people in their teens and twenties. Again, that is due to the maturation process. Sometimes relationships just run out of gas, and the end isn't influenced by anything external.
If this is the end, feel free to mourn it - it's a hard thing to take. But also understand that the world will not end and you will be okay.