How can you prepare turnips to make them less bitter?
Yeah, maybe you just had some bitter turnips.  It sounds like you did all the right things to hide the bitterness.

If you want to try a different cooking method, I tend to like turnip boiled and mashed.  If the turnip is old (and likely bitter) you can add an apple.  I've also read that you can stir in baking soda after the turnips have boiled to remove the bitterness. You would then need to rinse thoroughly. I've never tried this however.