Try doing rising trot without stirrups - make sure  you have a kind horse first though, some tend to rush as you bounce about. This exercise will teach you to really feel the movement of  the horse. Instead of rising right up on your feet as you would normally, just shove your pelvis forwards and up, then relax down (straight down - don't fall backwarsd) into the saddle. It helps if your horse has a smooth, slow trot. After a few minutes of getting a sore bum doing this, you'll feel it's a pleasure to go to normal sitting. Also try sitting for two beats and rising for two beats, instead of the usual up-down-up-down, do some up-up-down-down. This also helps with timing posture and balance - it's amazing what that exercise will do for your 'feel' and lower leg. Don't forget to breathe and be relaxed at all time. Don't be afraid to have a neck strap for some occasional support.

I would also try leaning forwards slightly as it will take some of your weight off of the horses back (: but I would also check if you are practicing for a test if you can do it in rising trot, as I know I find it easier to do, and I expect that you would too.