when you say you ride on your butt instead of your seatbones, do you mean that your pelvis is rolled back so you are on "your pockets", like as if you had jeans on with back pockets? Where is your upper body? Vertical or behind vertical? Do you have anyone to help you see this, or a mirror?
If you don't already do this, here's a starter excersize;
Sit on the saddle, legs dangling down, out of stirrups. Raise your legs way up in front til you can rest your feet on the side the horse's neck . YOu may need to steady yoursel a little by holding onto the back of the saddle. Sit as upright as you can, and your seatbones will now press STRAIGHT down into the saddle. Sit up tall and proud and without changeing your upper body or pelvis, let you legs drop down. Let them hang. This feeling of how your pelvis relates to the saddle is the place you want to be in during sitting trot . If it' hard for you to find this place or hard to keep it, it could be a number of reasons, including that the saddle is not balance well for you . Could also be that you are gripping up with your calves or are slouching ala cowboy style.
Riding sitting trot should be a bit like riding a wave. A surfer who rides a wave must stay on top or in front of the wave's movement. Once he is behind it, he is lost. YOu must be on top of or just in front of your horse's trot motion, which actually does have a certain wavelike feel to it. It is rythmic and moves not only up and down, but side to side also. If yo are behind the motion you end up having to hang on the reins to catch up.
Also when the horse goes down (in the landing phase of the trot) think of following him down . When he rises, you can be very passive and just let him lift you, then you kind of actively follow him down for the next bounce.
It gets easier with practice. I am no pro, especailly since I mostly post the trot. But I am learing how to post a few and sit a few , and again and then canter and then sit and then post, as is needed for my horse as he negotiates trails at canter and trot, as we do love to do.