Hey! Yes this is something we hear from a lot of players and it can be very frustrating for sure. It sounds to me like you have good foundations and are doing the right things, you just need some small changes to your training and I’m sure you can turn it around and make improvements. I would say you need to really work on a lot of irregular drills where you don’t know the ball placement and also a lot of serve and return drills to replicate match situations where you have to make decisions and quickly choose the correct shot.
Usually where you feel your shots are good and technique is strong but still aren’t getting results and improving it comes down to shot selection and being able to switch effectively between shots that you have practiced. This requires lots of random practice as I said and working on improving this decision making that will cut out these mistakes.
Also you mentioned that it’s mostly your mistakes where you lose points, so it is probably a case of trying to slow things down and focus on playing with good spin and control and not rushing to finish the points. It’s surprising sometimes just by being solid and consistent your opponents will be under more pressure and make more mistakes themselves. So focus on and work on having good consistency in your open up shots and the follow and not making mistakes here by going to fast. Work on spin and good placement, so into the body/elbow or wide angles.
I feel if you work on these areas you will start to see a difference in your results and improvements! Any questions on that just let me know