I had this same problem, I rode a hoooorrribly bouncy horse and nothing I did helped me to be able to sit his trot. My instructor finally decided I was going to learn to do it the hard way - so she took my saddle, tied my reins really short, and stuck two dressage whips through the crook of my arms and behind my back, so I had to sit up super straight and couldn't move my arms at all. It was like riding in a straight jacket. I thought that riding like that bareback was going to be the death of me, that one stride would launch me into the air and I wouldn't have a prayer of landing back on the horse's back. But somehow, it worked. It helped me feel what I should be doing with my body. My problem had been that I was collapsing my core on each bounce to try to absorb the shock (I have scoliosis so bad I wore a back brace for two years, so the bounces hurt my back), and by having my core so restricted I learned to go with the motion from my hips instead. Just really try to focus on sitting up straight and letting your hips move with the bounces, and most of all don't tense up :) Tensing up makes it a hundred times bouncier!
Another thing that helped me was to sit the trot for just three strides then stop, go three strides and then sit the trot again for three strides, and do that till I got comfortable, and then start upping the number of strides I sat trotting. That helps you not tense up because you only have to do three strides, it's a lot easier to stay relaxed for just three strides than it is to just try to make yourself relax while trotting as long as you can. Once you are able to stay relaxed for those three strides you can start going longer. I did that bareback, but I think doing it in the saddle would work just as well too. Hope that makes sense and helps some!