You've carefully laid out a systematic, progressive schooling program for your horse. Yet every time you add new work, you run into a certain amount of resistance.
Don't panic. Understand that when you raise the bar, it's inevitable that you're going to encounter resistance. It's a normal part of training. Don't be afraid of it. Just work through it in baby steps.
Have a checklist in your mind to help you decide if you should back off a little bit, or whether you should push through the resistance. There should be three things on your checklist.
1. Physical issues. You need to know that your horse is not in pain anywhere. Are his hocks are okay? Is his back is okay? Are his teeth okay? Do his saddle and bridle fit him correctly?
2. Check yourself. Make sure you're giving the aids correctly. You want to be sure that you aren't giving contradictory signals.
For example, let's say you're riding to the right (Your right leg is on the inside). You turn down the quarter in order to leg yield over to the left.
Your right leg is behind the girth asking the horse to move over. However, you have a very bad habit of pushing too hard with your right leg so your upper body leans to the right. Since your horse wants you to stay centered above him, he finds it hard to go sideways to the left because your leg is saying, "go to the left", but your body weight is saying, "I won't let you go to the left."
Then you end up thinking, "Oh, my horse can't go sideways. He's not ready. He´s resisting." But the reality is that you're giving conflicting signals.
3. The third thing that I do if my horse is really showing me, or telling me with his body language that he can't do something is that I find a way to take the difficulty out of the exercise. That is, I do the "essence" of the exercise, but I make it more simple.
Here are some ideas so you can be your own problem solver and figure out how to take the difficulty out of exercises but still get your point across. If you take this approach, the resistance becomes manageable or even nonexistent. Then little by little, you can increase the demands again.
For example, let's say you start to leg yield from the quarter line over to the long side, The first few steps are fine, but then your horse starts resisting. Maybe he slows down or tosses his head. Take the difficulty out of the leg yield by starting only 1-meter off the rail instead of from the quarter line.
Or let's say you're struggling when you start your advanced lateral work such as shoulder-in, haunches-in, and half pass.
There are several things you can do. You can reduce the angle. Rather than asking for shoulder-in, do shoulder-fore (half the angle of a shoulder in). Or rather than asking for a 3-track haunches-in, ask for half that angle. With your half pass, rather than going from the corner letter to X, reduce the angle by going from the K or F all the way up to G.
Regarding shoulder-in and haunches-in, do fewer steps. That is, do three or four quality steps, and then straighten your horse. Let him take a breath. Then do three or four steps again. Or do the movements at a slower gait such as the walk.
Just be very clever on taking the difficulty out of the exercise. Introduce new work in baby steps so that your horse always thinks he's a champion no matter what you're asking him to do.