The best thing you can do for yourself is first figure out what's causing the block. What is it you're learning? Is it a specific thing or several? Is it the horse or instructor? Is it you? Are you nervous? Do you get flustered easily and has this always been the case? If not, what triggered it? Did you have a fall?
You don't have to answer these here but they might be helpful in figuring out why you feel you aren't improving, if you don't already know. When you figure out why, then you can move on to coming up with solutions