Ya, some horses are VERY hard to sit too.
There is this kinda fat mare, that has an easy canter to sit to, but ehn, i went to an appy gelding, who is bouncy, and sometimes spazzy, and continued to learn to canter on him, eek!!!!
Now, i have no problem sitting to his canter, and even sitting trot, not everyone can sit to him!! its likr riding a 2X4 my instructor says, lol.
Dont worry, youll get it!!!! i did after at least a year of riding, and doing no stirrups work helps a lot (yuck, i know, i dont like rising trot with no stirrups!!)
BUT, since i did a lot of work at trot, and with no stirrups before i started cantering a lot, i think it took less time to get used to sitting to it, im actually better cantering without stirrups than with, because i dont depend on them.
Wel,, ive blabbered on AGAIN, so TTYL!