That gives me hope. I have been battling with the sitting trot. Huey has a GIGANTIC movement. We tried trotting with no stirrups, and it took three paces to bounce me right out of the saddle, through the fence, and onto the ground. I have started to despair that I will never be able to do this. :(
I definitely know how you feel.. but there is hope yet! I tried the whole no stirrups thing and I was slamming on my horse and so I just decided not to try it again. So I learned how to sit trot with my feet in the stirrups.
Think of your feet as shock absorbers.. your upper body usually gets propelled UP (posting trot) but you are trying to stay DOWN (sitting trot) so you need to focus on staying DOWN.. pinching with your knees isn't going to keep you down.. it's like a spring.. you'll fly up and then gravity will bring you down :P think about relaxing your knees, pulling your weight down into your heels, keeping your legs on the horse (not squeezing.. just there to keep horse moving by controlling the feet..) But not dangling 100% either. It's a fine balance. And you'll get it! My horse's trot when I first got on felt like I was in a speeding car or on a rocket ship! It's HUUUUGE! But as soon as I mastered posting and then practiced my walk to trot transitions.. I snuck some sitting trot beats in there and pretty soon I was doing it.
But begin slowly!!!!! Get your horse down to the slowest trot, maybe even a jog.. so you can quieten your upper body and feel what it feels like to get it right.
I read all of this thread and I know it is all good advice, but there is no way I can think of all these things at the same time! I am hoping it is like posting the trot and it is one of those things that seems impossible until you suddenly "get" it and then it makes a ton of sense...
Oh but of course.. however your body is always making microscopic corrections as it learns what works and what doesn't. Miles in the saddle, my friend.. and it'll become easy.
And don't get me wrong, I can do a sitting trot but not at the working trot speed/power yet :P We're at a slow jog and gentle trot but it's something!!