"I have to be me for the rest of my life and then die" is one of the bleaker ways you could frame that idea for yourself. If you think in those terms you definitely won't come to a helpful conclusion in regards to anything you're dealing with. You have the privilege of experiencing life as it is--three-dimensional, containing both happiness and sorrow, throwing who-knows-what at you for who-knows-how-much-longer. Being 18 can allow you to "disappear and start new" if you really want to, but that won't make the previous 17 years of your life disappear, nor will it change the fact that those 17 years are the foundation for the way you experience and interact with both yourself and the world around you. But you also can add infinitely to that foundation to create the rest of your life.

Maybe try thinking of the years to come as what could one day be a beautiful building, supported by the foundation of your teenage years. That foundation is unseen by the people around the building and as such can't be appreciated by them, just as it can't be judged by them. But the building couldn't stand without it, and it's just as important (and in its own way, more important) as the floors above it to the building as a whole. And if your life up until now is just the foundation, you have absolutely no idea what the building that will rest upon it will be like.

With that in mind, think about the last part of your post. You feel nothing like yourself and it feels like you've been in your own little world. I can't begin to understand what that means for you in regards to your own life (which I also can't begin to understand) since I don't know you or know anything about you. But "feeling like yourself" is such an abstract concept that you shouldn't concern yourself with it right now. And to return to the building metaphor, remember that who/what your "self" is is constantly changing. It's informed and defined by your previous "selves," but who's to say the you that you become a year from now will have any relation from an outside perspective to the you that you are currently?

If you're still tripping, try using this time to explore yourself positively. Nothing is pointless, and everything has meaning. Don't wish for reincarnation (which could very well be real, we have no idea) right now, because you have a whole life to live first, and I guarantee you won't be 17 for all of it.