Mara, honey . . . well, first of all, congratulations and a giant hug!
You have everything to look forward to! It might seem like the end of the world, but I promise - genuinely promise - that life will begin to get better. You'll have your ups and downs (and plenty of downs, but the ups are so awesome that it's sooo worth it), and we're all here to help in any way we possibly can.
Many of us still wonder whether this is just in our heads, that we've made it all up and we're being misled by our feelings. It's good to be uncertain at times; it makes us really think about who we are and whether we're making the right choices. I was only last week talking to my therapist about whether I was just a crazy person pretending to be trans. (Answer - no, and it's a very common doubt.)
Even if you're still unsure, give yourself permission to experiment a little. Assume that you are trans and go from there. Grow out your hair. Change your style. Start playing with makeup and clothing.
Talk to a therapist. Nobody - I mean
nobody - will ever have to find out, and you can see whether those kinds of things bring you some relief. It's not as if you have to tell the whole world tomorrow morning that you're trans. You have so much time to get things ready behind the scenes, so much time to practice and figure out in your mind that you truly are transgender, and there's no rush to do anything. But please, do something - don't bottle this up any longer.
You haven't ruined your life. You've figured out what's wrong, you've found the solution, and you have everything going for you right now. You're young enough that there's no reason to believe that you won't be utterly successful in this, and in a few years you'll be an absolutely normal-looking young woman living the absolutely normal life you want to live.
I'm excited for you. You should be too. It's absolutely scary and confusing, but soon it'll feel normal in such a good way.
Another massive hug - you deserve it. Now go finish the last of the wine and smile!