When something isn't working out quite like it should, I'll often hear from a student, "I'm doing what I'm supposed to be doing. I have this rein here like I should and this leg there..." Riders can make dressage an all too intellectual endeavour. My most accomplished students have been young people who likely never read two sentences on any theory or about any master. You know why? They are in the present feeling. Like any discipline, a well educated mind and body are the ideal combination, but what you know in your head isn't worth anything if you're not aware of what is actually happening. So how do you apply what you're learning? Experiment. As my dad says, the so-called aid is only an aid if it is aiding you.
Lateral movements can often be a paralyzing zone for riders. One direction comes with ease, but the exact same aids going in the other direction result in no lateral movement at all. There are a number of reasons why this is happening, but the simplest is often the basic fact that what one does easily to the right is not as easily done to the left or vice versa. Try writing with your opposite hand. You likely don't even hold the pen the same way. When it comes to your horse, he has his own asymmetry to deal with and then there you are plopped on his back with yours. When approaching a lateral movement to the "hard" side, take it down to its simplest form and then move up.
For example, your half pass left is non-existent compared to the right. First, take a leg-yielding left along the wall or fence. You'll have the support of the architecture and an easier lateral movement. Slowly begin to change the bend in order for your leg-yield to begin to transform into a travers. Then take your travers into a large circle. If that goes well, transition from the travers in the circle into a half pass on a diagonal line. Take breaks intermittently when you've arrived on a good note to avoid hammering out the exercise. Be patient. It's easier said than done - I know.
The last ingredient is having some guts. Ask for what you want with a sense of conviction with a good dose of kindness. Try something you don't think you can accomplish. Any personal trainer I've ever known never got the results out of their clients by letting them workout however they choose ;)