One mistake people often make when they ride a horse that's running away under them is to take off their leg because they don't want to encourage it. It really helps to actually do the opposite. Try to slow down your posting but still put your leg on everytime you sit down. The trick is to get the horse to accept your rhythm.
I have the same problems with my horse. I did lots of trotting poles and sometimes if he wanted to run I'd just canter a few laps to get it out of him. Now he's improved immensely. You just have to retrain him to think slow. Trotting poles are very helpful and 10m and 20m circles and serpentines to get him thinking and listening
Hey😊 Something I find very helpful for slowing the trot would be to slow your rising in rising trot. I find it so helpful to count in my head this helps with my pace and balance. You can also do some pole work exercise. Hope this helps😊
Pretend your body is frozen while riding. Don't actually sit there frozen but keep your legs not moving and half halt. If your horse doesn't listen, half halt a little harder. I have this same problem with my gelding. My instructor says that I should pretend there's a spider under my foot in the stirrup and to press down hard and crush it.