I just read a post on TeamLiquid.Net that reminded me why one shouldn't get so nervous on ladder games - go in with the expectation that you are going to lose. No wait, lose and improve. Yes, you can get higher in the ladder by following your same, 80% success-rate build order, but chances are you won't learn anything, and you won't ever be able to counter those one or two builds that beat yours. You may even encounter a single race that you can never beat, even if you regularly stomp the other two races.
Experiment with build orders. Do some crazy things. Try attacking at different supply milestones. Try seemingly stupid things you read on TeamLiquid. Try copying a pro's build. Try practising your marine-micro, or dropping banelings from Overlords on TOP of marines! (What fun!)
So... lose, and keep losing. Keep learning.