Hey there! I have had a similar problem with my 26 year old half arab. I started ridding her six years ago. I had a very difficult time getting her to soften/round and going slow in trot or canter was out of the question. Then it was recommended to me that I look up a man named Philippe Karl. His work is amazing!!! I am fortunate to have a trainer in my area who attended his three year program, but I also read his book and it helped immeasurably. It is called twisted truths of modern dressage. The name is a bit off putting but the information is gold. We’ve gone from not being able to trot without bracing and running to doing second level movements in a relaxed round frame. I hope this helps you as much as it helped me.