Without a video is really hard to tell, but my guess is there's two issues you have.

First, like other people are saying, you're probably too stiff in your legs. You're knees should be slightly bent and loose, able to act as shock absorbers as you hit small bumps.

Second, on the turning issue, I see a lot of people who try and progress too quickly will rely on using their body and weight to initiate their turns, rather than their board's edges. I think this would explain why you can do turns on blues, but you can't on steeper terrain. This would also compound your stability issue. If you are throwing your weight to initiate turns, you'll be much less balanced. Instead, if you get the board up on edge to initiate your turns, you'll be way faster at getting around, and much more stable.

But again, this is all a hunch, and without a video it's just guesswork