When you are told to feel the movement, it is not the bounce you are meant to feel. If you sit on the horse in walk you should feel your hips dip in turn as his hind feet move. Breathe slow and deep and relax into that feel. If you can, find someone to lunge you or give you a lunge lesson so you can shut your eyes if you want.
The good news is that the movement of those hind legs in trot is rthe same as in walk.
POn some horses it is easy to feel and let your seat bones did with the trot.
But on a bouncy horse, the abrupt up down movement and being precarious invades your brain and you stiffen. On a very bouncy trotter, I need to relax and even shut my eyes to "get" the side to side movment, but once you have got it into your head, your body should pick up the feel and allow it to happen under you. Breathe and relax and allow your seat bones to dip with the movment, first one side and then the other, just as you did in walk.
Then you should have the same sort of feel as you had in walk.
If you dont get the feel of it it fairly soon, I think it is better to come back to walk, (in order not to get bounced and unstable in trot) and ask for the transition again - at least that is how I was taught to trot bareback.
But once you get the feel, it is like riding a bike. I dont forget (my body doesnt forget) from one summer to the next.
Just one more point. Some people find it harder on one rein than on the other, and a horse may be smoother on one rein than the other. So if you have a problem going one way, try it in the other direction. I learned to trot bareback on the right rein a whole week before I learned to do it on the left rein.
I hope this helps. Those of us who learned sitting trot from the start find the rising trot difficult!