Loner, I hope you take solace in knowing you're not the only one who has this problem. It is, in fact, why I've not actually finished a book. The plot takes control of the story.
There is, however, a remedy for this, I've learned. It takes some work, but, it DOES work, the point being. First, you take your outline, and then you write the story, action for action, as if you were dumbing it down into bedtime-story form. Once you've finished that, you've successfully gotten the ball rolling. Once I get these fairytale versions of my stories done, I go back through, flesh out events I haven't completely elaborated upon, and before I know it, I've got the story I wanted, plot in tact.
I hope that helped some. It's not an inviting idea, but again, it's one that works.