What I did to help my sitting trot was grab a little piece of mane to steady myself at first (there's no shame in it! After some practice, grabbing the mane wont be necessary, but its better than tugging on the mouth!). I worked on relaxing my lower back and letting my body move with the horse more. I worked without stirrups a WHOLE lot and only practiced without them. Finally, with the stirrups, my position was 110% better. It also helps to keep the horse collected and slow at first. Once you get better, it may be easier to go faster. Good Luck!
“Have fun - Stay on top.”
"There's nothing like sixteen hands between your legs"