How does one go about changing bad technique? Three recommendations.

First, exaggerate the proper technique. If you don't rotate your shoulders enough on a shot, practice over-rotating until it becomes comfortable to do it the proper way.

Second, watch top players (live or on video) who do the stroke well, and visualize yourself doing it that way. The more you visualize it done properly, the more likely you'll do it properly. Then shadow stroke it as you visualize it.

Third, drop out of tournaments and match play for a while and focus on fixing the technique. Perhaps hit with a coach a lot for an extended periods as you fix the technique. Playing matches will just reinforce the bad technique. If your goal is to really overcome poor technique and replace it with good technique, then you need to have an extended period where you focus on this. That means only playing with the coach, or doing drills where you can isolate the new technique so you can focus on doing it correctly.

You should be able to play without the coach as long as you keep your outside drills simple and focused in this way. You might also want to use videotape to verify you are doing it correctly when the coach is not around. Bad habits are not easy to change, but if you really want to change them, you need a very focused period of time to do so.

In general it's best to play lots of matches and get as much tournament competition as possible when trying to improve (along with lots of regular practice, i.e. drills), but when you are making major changes to your game, it's often best to take time off from competition. Perhaps make a goal to have your game ready for tournament competition for a specific tournament (or series of tournaments) six months or so away, and train specifically for that. I don't think you need to take six months off from playing practice matches, but perhaps two months off would greatly help you in making these technique changes.