Have you ever been writing the plot outline, or even a chapter in one of your books, when-GAH! You realize the plot is supposed to go a different way, no matter how great your first idea was, and you have to re-write the whole thing. If you've experienced this, what did you do about it?
That pretty much sums up the process for half the scenes I write. Sometimes I realize midway through that it's not working, otherwise it's not till I read it back later and think "nope, I should do THIS."
But that's okay. Writing is re-writing. You don't have to get it perfect the first time. You just have to keep trying and polishing and pushing yourself until you get there.