I don't post here much, and for good reasons. But you know what, I've been where you're at. Too many times. You like a girl, she's going into the Military. This is a fact you need to face because your paths are going different ways. This girl, is apparently so great but you can't get over the past. She clearly has, shit happens dude.
Alright, so you're paths are changing that's life. People come and go. But from reading your post, man. You have some work to do. I'm talking about finding your self. You put on a mask for different people in different classes. Fuck that noise. If people don't like you for you, fuck em', but don't cut your self short by not being you. In this world, not everyone is going to like you, I love this fact because I fucking don't like everyone either. It'd be absurd if we did right? I don't need someone to tell me I'm cool, interesting, whatever. Because I know I am those things! You are too, don't ever forget that. Believe in your self, or like they say "Fake it till you make it."
A couple of things you should work on:
Also, walk in your graduation because you only get one chance, and when you see this girl, tell her how excited you are to be starting a new chapter in your life!
Something I wish the younger me knew in high school is you're going to go out into the world, and meet some probably amazing people that will change your life.
Something I can't make you understand buddy, but you'll meet another girl who you'll feel this way about. You might not want to believe it, but it's the truth. Your paths are diverging, and life is happening. It's hard to swallow sometimes, but you can do it!
Post-Graduation is a lot of fun, work and new experiences, it's super exciting so be excited for what the future holds and don't be afraid! We play the hand we're dealt and that's that.