Try bareback lunge lessons using a vaulting surcingle, if someone has one. You hold onto the handles, and just close your eyes*
, relax, breathe. Start at a walk and just feel out the horses movement. And as you pick up the trot, concentrate on dropping the front of your pelvis with each step. Having your instructor just cadence out when to drop your pelvis gives you an idea of the timing as you learn the feel of it.
*important, keep your eyes close, because your sight does distract you. You don't have to worry though, you are on a lunge line, and not going anywhere but in a circle. It's pretty cool to experience too.
At first, you use the surcingle to pull yourself to the horses back, forcing yourself down. You'll actually be pulling yourself into the horse, so you won't be going anywhere lol. As you get more confident, relax your hold as your body learns to follow the horse. And keep practicing until you can ride the trot without holding onto the surcingle.
Your arms will hurt. Your abs will hurt. Tough it out.
I had a great improvement after just one half hour lesson like this. My instructor only does half hour lunge lessons because they take so much out of horse and rider trotting or cantering in constant circles.