Don't listen to your coach !! Only joking .

It's very difficult when you have a swing altered from what feels comfortable to you. It can throw your game right out the window, which it appears to have done. Looking at your profile, I see you do not have a handicap, so I do not know at what level you are at. So when you say you were hitting the best you have ever hit, what was different?

Your coach has given you good advice, and if in practice it was working, then it is unlikely that you are transferring the new technique to the golf course. This may be for the very simple reason that you are worrying about all the new instruction, that you are tensing up and forgetting to relax and enjoy the game. A swing lasts a couple of seconds, and this is not the time to be worrying where the club head is pointing. As long as your grip & set up are fine, rotate the hips and shoulders and swing comfortably. If you are trying to check each part of the back swing as you do it, you are more likely than not swaying the hips.

Cheers.
Lee.