I have been a student of the art of learning for many, many years. It fascinates me! I do not teach players how to learn tennis. I teach players how to learn to learn tennis! My understanding of the learning process was born of years of curiosity about the process, teaching experience, studying coaches and athletes, trial and error, and being under good leadership.
After working with students for over 30 years my conclusion is that most people are just not in tune to learning and all the myriad phases and complexities involved in learning. Take for instance intellectual knowledge (knowledge you can understand and repeat back) versus application knowledge (knowledge you can apply automatically). Players constantly mix them up. I have been told many times, "I know that already, why do I have to do it again?" My answer is always the same. You may know it but you are NOT doing it. Until you can do something automatically in application form you do NOT own that shot, stroke, etc. Application form means applying the technique instinctively in match play, not just in practice.
You must integrate what you learn in practice into your matches. Just because you can perform a technique in practice does NOT mean you can perform that technique in match play. You must slowly integrate that technique into your match play by continuing to practice until it happens automatically in your match play. If you can perform a stroke only in your practice then you do NOT have application knowledge yet. But if you are patient and continue practicing, the technique will come around in your match play. Just relax and keep up the repetition.
Here is an example:
At my club I was coaching one of the pros who was having a huge breakthrough in application knowledge. One day, after months and months of practice on his forehand and backhand groundstrokes, he got on the court to practice and out came these fluid, beautifully balanced, well-timed strokes. With renewed optimism, he thought he had arrived! Or had he? I explained, to no avail, that this is only the genesis of true success. There is still a ways to go to own these strokes.
Another pro at my club has won every tournament around. He is a tough, resourceful player. The pro I coach often plays him to test how well he is performing. He loses every time, but has improved to 6-4 and even 7-5 set scores. Well, after this breakthrough he thought, "I'm ready to take on the best, right now!" After his practice and a word of caution from me, he went off to find the other pro. Unfortunately he found him! They hopped on a court and played a set. The result: 6-1. My student went down in flames! He overplayed everything trying to emulate how he had performed in practice. He came back disheartened and discouraged, but still eager to get back to work and own the new strokes he had the privilege of briefly experiencing.
The point is you learn in increments and baby steps. There are no exceptions. Everyone learns in increments. Once you have learned a stroke or technique you MUST take that technique to an extremely high level of development in practice before you can apply it in a match. Usually, with enough practice, the application in a match happens by itself - automatically. A phrase I like to use to describe this concept is, "when the student is ready the stroke will appear!"
To be able to apply a technique from practice to match play, your rate of learning must exceed your rate of forgetting. When you are in the beginning stages of learning a technique your rate of forgetting exceeds your rate of learning. You must keep practicing to allow the natural learning process to mature. One day the rate of learning will exceed the rate of forgetting and bingo! The stroke or technique will be all yours in a match.
Learning is tricky business. You must slowly prepare your mind and body to accept new information, and then once this information is learned, integrate it into your body's automatic nature.
Stop being frustrated by attempting to use what you have just learned too soon. Remember to learn in increments and do NOT forget that integrating what you learned in practice into your match play is one of those increments!