Hi & welcome jmg12287
I'm sorry but Argyll is right, this sort of thing happens all too often. Couples pair up when they are not much more than children and stay together until they are adults ~ but adults and children may be very different people & when they start to experience the big wide world they realise that this childhood / teenage relationship no longer feels right.
Sometimes it does last, but more often it doesn't. Maybe she would rather just be friends, without the pressure of having to be a couple.
But, while this must feel bad, you will find that it was for the best, because you would not want to stay with someone with whom you weren't compatible and who was wanting to experience life beyond your close relationship.
In a few years, when you are older and settled, you will look back on this time with fondness and you will most probably be feeling very happy that this girl set you free to find your real life-long soul-mate.
Good luck!
