Ask someone to help you with this, as in hold a lunge line so that you don't vanish into the distance!
Take your stirrups away - it means you cannot brace your feet against the stirrup. Shut your eyes - this allows you to be moer specific when you focus on the movement. Takes away all visual distrations.
On the lunge in Walk, shut your eyes and totally focus on the movement of the horses hind quarters and allow you bottom to move softly with the horses movement.
Now move on into a very slow trot EYES SHUT! Still focusing on the movement of th ehorses hind legs. Keep the trot slow while you discover the feeling. If you start to bounce go back to walk and get your balance back.
What you should feel is your hips being moved in two different directions in two different planes!
One hip will go forward and up as the othe comes back and down - very much like the movement your hips make when you ride a bicycle - in fact a quick spin around on a bike will give you the general idea. Practise in a slow trot and always going straight back to walk if you start to bounce. As you get more feel and suppleness you can increase the trot. Once you get the hang of this with no stirrups then take them back - sitting trot with stirrups is much harder.
In time your sitting trot will become easier.