Playing in the woods and playing in the open are two totally different skills.
On open courses, what matters is the destination, not the journey. It doesn't matter how you get it there, as long as it lands where you want it to. In the woods, it's the complete opposite. The line is what matters more than the final destination.
You're accustomed to just ripping it and not caring about minute details of the release that may affect where the disc goes. Once you get into the woods, those minute details are hugely important.
The best way to learn to throw in the woods is really to start over from the beginning. Don't worry about how far you're throwing. It sounds like you're still trying to crush big drives, stop that. Disc down, take out your run-up and just try to hit lines. You'll find that staying in the fairway 100' short of your target is far better than crushing it into a tree and kicking into no-man's land. As your line-shaping skills develop, you can start focusing on hitting those lines for better distance, but take it one step at a time.
Amateur Challenge Tournament - Wickham Park - Manchester, CT - June 1, 2013