♪Other fish in the sea, other fish in the seaaa♪
Ahem, sorry about that. You could use a laugh.
Before I saw anything, maybe you should clarify more. How exactly have you been in love with her your entire life? Have you had her for all your classes? Have you always known each other but you never made a mood? Regardless, that's no reason for you to feel heartbroken. You should have been paying attention to other girls as well. Trying to narrow your target down to one girl may seem more reasonable at first, but then you are left feeling just like how you are feeling now when you find out she's not interested/has a bf because you have been glorifying this one girl the whole time.
In conclusion, just remember that there are always plenty of opportunities to get a girl, but it's up to you to enable how many of those opportunities come to you, meaning go out of your way to meet new people as well as befriend or get to know more girls. Don't be shy!