Tape your lessons.. and when you have it down right, then study it. Study it a lot.. remember what it felt like when it was right and concentrate on that feeling.
Write it down in words, like describe what it felt like and HOW you got it.
Trust me, that is how I figured out leads of a canter quickly and also the sitting trot.
Then practice trying to get that same feeling. Reward your horse once they get it and be sure to take lots of breaks. When you work on something you and your horse aren't 100% about, it can be a LOT of work and sometimes we focus too much on getting the results and forget about how much effort it takes us to get there.
So tape, review, describe, feel, and take lots of breaks.
"Strength is the ability to use a muscle without tension"