Once upon a time, I was a green rider with a horse that was capable of a canter-walk transition and of working in a Second-Level frame, if you gave him a strong and clear aid: slowing and half-halt, respectively. The problem was I didn't know how to do that. Try as I did, my aids were a bracing against the motion and not only didn't slow or rebalance him, but they certainly unbalanced me. It was a move similar to what your body might do right before crashing your bicycle. Every day I would practice, with little result.
Then one day we went to a show. As I piloted this monster of a horse around a crazy warm-up, what seemed like every Grand Prix stallion in the area suddenly materialized. As I cantered around the corner, one of them turned the corner in front of me as they headed for a diagonal line of tempis. At the same moment, another stopped in front of me. While I am not certain they were all massive stallions, I am positive I went from canter to halt in a split-second. I had found my rebalancing and slowing aids! Instead of feeling like I needed to stop from crashing, I felt like WE needed to stop from crashing. That feeling was my first realization that when our aids for the downward transition and/or half-halt aren't working, it helps to check to see if you are trying to rebalance yourself AND your horse.
Avoid the misconception that you need to change your body and take the horse with you--plug into your horse and change his body, as a result your body will follow naturally because you are sitting on top of him! It sounds tricky, but if you ride your "learning" canter-walk transitions and half-halts like you are trying to stop your horse from falling off a cliff, you will hopefully get the hang of it. The good news is that once you have this exaggerated version down, you can play with the various types of slowing/rebalancing aids and half-halts that dressage has to offer. Remember: exaggerate to train and then refine to master. Happy riding!