If your parents can't pay for lessons for you to learn to post, you could do one of two things:
1. Watch some instructional Youtube videos on posting.
2. Sit the trot, take your feet out of the stirrups and learn to sit back on your seat; I'd bet money you are way over working your legs/knees/ankles when you don't have to. Also, slow her down to a jog if her trot is that rough
Once you master the trot, then move on to the canter