[Eternal]

Okay I read the whole thing and first, let me tell you that you have an excellent voice when writing.
I know how it feels to walk across the stage and not feel ready. I still had crippling social anxiety and depression when I graduated high school, and really it didn't go away until this year. Even if you don't feel ready, that doesn't' mean you won't ever be. Honestly going to college, whichever one you end up at, will give you those life skills that you just can't obtain by being trapped in a building all day and then going home to your parents. So don't fret too much, you will be okay and you will gain life experience as you grow older.
I also want to note that it wouldn't just be your teachers and your dad who would be sad if you were to die. When I was in high school, we had multiple suicides throughout my years there. And it affected everyone, whether you just saw them in the hallway or had even just heart of them that day. When you are depressed, death does not seem like a big deal. It just seems like the eternal peace you dream of. But to everyone else, to know someone their own age, young, had their life cut short? It's really grim. Everyone at your school who knows you, whether you see them as a friend or not, will grieve and think about you. And unfortunately yes, they will think to themselves "I wish I had talked to her more" when really we should be actively trying to be kind to everyone. Nobody realizes the pain others feel until it is too late often times. Additionally, your half siblings would not just roll their eyes. They care about you more than they show, I promise you. They will miss your presence and all the wonderful quirks you have that you don't even know you possess. I know you feel invisible and unimportant but you are not.
As for your jealousy, that is understandable. But it is not your fault for not also going to an elite school if your parents do not have the money for that. And evidently it isn't their fault either, it's the government for making higher education so expensive that only around 1% of the world population can go to college. So getting into elite schools for your grades is amazing, and you should feel more proud of that even if you do not plan to go. Going to a four year public university is not so bad either. Your education may not be the same as your boyfriend's, but it's likely you will be studying different things so what does it matter? Most information you learn in college can be obtained through the Internet or textbooks anyway. Not going to an elite school doesn't mean you can't learn as much as him.
You seem to have low self-esteem so remember your boyfriend chose you out of any girl he knows. He likes and wants you. Going to different schools can be scary because of the fear he will find someone else, but honestly if he leaves you then that is on him. You can easily stay in touch every day and video chat or have phone calls and you will see each other on holidays. Long distance relationships can be very stable, they don't have to end unless you don't put in any effort.
Everything will be okay. This is a very tough point in your life, but I promise you will figure things out. Doing well in school is setting you up for great things in life, and whatever is bothering you now will go away in time. You're going to make so many great memories in college, and I promise it will be worthwhile even if your boyfriend is not there. So far college has been such a better experience for me than high school, and just about anybody who also has gone to college will agree with me. So keep your head up.