How does one go about changing bad technique? Most players use halfway measures, and when that doesn't work, they give up and go back to their old habits. They may try minor adjustments when a major one is needed. They may change from one poor technique to another. They may practice it properly, but then, before the proper technique is ingrained, they'll play competitive matches and fall back into old habits, thereby re-ingraining the poor technique. They may have an improper grip or stance which causes the poor technique. Or they simply don't know what needs to be fixed. How can you overcome this? Here are five recommendations.
First, drop out of tournaments and match play for a while and focus on fixing the technique. Hit regularly with a coach or practice partner 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, i.e. saturation training. That means only playing with a coach or practice partner, and doing drills where you can isolate the new technique so you can focus on doing it correctly.
Second, 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. Shadow practice the proper technique over and Over and OVER until you can do it in your sleep, on your deathbed, and most importantly, at deuce in the fifth.
Third, 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.
Fourth, make sure your grip and stance are correct. If you get these two correct, then everything in between tends to fall into place. If you get one of them wrong, then fixing a problem somewhere else won't work unless you fix the root cause of the problem - the grip or stance.
Fifth, work with a coach. Fixing bad technique is his job. Let him do his job.
Let me emphasize item #1 above. 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 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 of focused practice without matches would greatly help you in making these technique changes.