Directionally Challenged?

Have you ever met anyone who is really bad with driving directions? Well I am not one of those people. Sometimes it almost seems I was born with a built-in GPS, whereby I can tell approximately where I am and how to get where I'm going. I always say, give me a map and I can get there.
But when it comes to writing it seems like a whole other story. I set out to write something and somehow, somewhere along the way my fingers will take over and the next thing I know, I've written something entirely different than I planned on. Sometimes it comes out sounding really good, too. But it's just a whole other direction than I had planned on going. Needless to say, this makes book-writing a little difficult. I know what I want to say and how I plan to get there, but once I get to writing, who knows where I'll go!
I find this to be especially true when I'm writing about complex, multi-faceted subjects. I'll take one branch of the subject, but then I can't seem to figure out how to tie back into the original root and take on another branch. It's quite frustrating and makes writing much more of a challenge, but I don't know how to fix it? Does anybody else have this problem?