I know the feeling, though for a slightly different thing. I started planning my life when I was 11 and decided I was going to be a biochemist and get a PhD. I was second in my high school class, I graduated university magna cum laude with a B.S. in biochemistry and a minor in neuroscience, and I moved to to Scotland to do my PhD. I even met a Scottish boy to marry! Then I finished my PhD. Wrote my thesis, had my viva, and finally graduated. It was a wonderful feeling, but then I realised that I didn't have a plan for after that, and I've felt slightly adrift ever since then (3 years now!). I've decided I don't want a normal academic career, but I don't know what I DO want to do, and that's pretty scary for someone who spent 17 years knowing exactly what she wanted to do and doing it!
It does get better though, and you won't feel low forever. You just have to give yourself time to adjust to normality rather than the active goal-seeking that has driven you!