I know what you mean with the twisting. I get that sometimes. If you feel it diving when you pull on your paddle, stop and setup again.
The problem you are describing is that you are panicing a bit and you're not getting the roll. Speaking from experience, I can tell you that the only way to overcome this is PRACTISE! Keep rolling in flat water until you feel confident and you're not doing silly things (not setting up properly, paddle diving etc.). Once you have that sorted, roll in moving water (make sure it's deep enough). This helps your confidence a LOT when you roll in a rapid. You'll find doing this that when you roll over in fast moving water you'll be relaxed. Because you're relaxed you wont have any troubles getting into your setup position.
But make sure you practice and get your confidence up. The longer you leave it the more you'll lose confidence and the harder it will be to get it back! Good Luck!