She's 15 years old. In one, two, five years.. she won't be the same person as she is now. Heck in a couple of years you probably won't be the same person you are now, either.
You need to let her grow up. Heck, you have to grow up.Did you know the center of your brain responsible for understanding long-term consequences and decisions does not fully mature until you are between 20 and 25? People thinking they've met "the one" when they're under 18 years old.. well I hate to be blunt but they're almost always kidding themselves. Don't get me wrong, there are definitely some people out there who end up marrying their highschool sweethearts, but they are vastly in the minority, if not a rarity.
There will be many girls and many friends and many things in your life to come. Enjoy her company and her heart for God, but understand you have a lot of growing to do yet and a lot to experience in life, but she has even more to do than you.
Enjoy life, hobbies, education, friends, family, pets, church, God, and so on.
Relax.