Try to do the transitions stirrupless. And always think tall and shoulders back, pretend like there is someone pulling a string up from your head making you sit up.
Sit up straight and try rolling your hips forward and under you it's the best way, I taught my friend today how to do a sitting trot(she's never trotted before and never ridden English before AND has never ridden my horse before) and I had her start trotting and once I told her to roll her hips forward and under her her sitting trot was great, so try that and then just ask with a lot of outside leg
Ask your trainer if you can sit trot over poles. That does makes it bumpier, bad but it works so when you are done with the poles, it would probably be a bit smoother. That's what i did and it worked on three horses and some lesson kids
Hey, I'm not a pro but, I pretend I'm like a ice cream scoup (weird ik) and really push my bum into the saddle, and put my innner leg slightly forward and outer backwards cause it helps my pony get on the right leg and a nice forward canter! X