Ride without stirrups on a trusty lesson horse, and have your instructor control where the horse goes. It's best to lunge, but I use to drive my lesson horses forward when my students where learning their aids, and the horse was either insensitive or just taking babysitting care of the rider. This IS what you want from a lesson horse, btw.
It's going to take you thousands of hours to be immediately comfortable at the trot, so don't expect too much, too soon. I found that the more athletic my student, the faster he/she picked up a good seat, so cross-train and develop your core and your legs, too.