What are some advice to have a straight back and to use your hips while cantering? My horse has a canter that is hard to sit. To absorb the motion I round my back and ride it with that motion.

Roll your shoulders back and try to think about making your shoulder blades touch the mid should straighten you up and give you a nice position. Hope this helps 😊
I agree with @mrd1 to do lots of no stirrup work in walk and trot, this will enable you to engage your core muscles and use your seat and get a better balance. I’d recommend going back to sitting trot, the reason you can only absorb the motion by rounding your back is because you are usually your back to absorb the motion instead of your seat, this could be due to a fairly stiff hip or not enough practice at getting your hips to become independent from the rest of your body. Sitting trot have yourself relax but pull yourself upwards so your straight, when going into sitting trot your hips want to be supple and be able to “fold” when the horses back comes up, imagine your belly button opening and closing. When asking for canter, do sitting trot first as that way you already have the seat when going into canter (sometimes it can be lost during the transition), but sit upright and think about sweeping with your bum, you might also be very stiff in canter which would make it difficult to loosen up and just use your hips, but everything comes with practice x
Hope this helps x
Practice getting a nice position by doing no stirrup work and bareback riding. These both encourage you to have a good position, because you need to keep balanced so you don’t fall off (because obviously you don’t have the saddle there to help you). Make sure you have a good position in trot, walk, halt.
As for sitting in the saddle, pretend that your bum is glued to the saddle and the pockets of your joddies need to be touching the saddle at all times. You can practice sitting trot and if you get it in sitting trot then it will probably be easier in canter. Sometimes when people aren’t sitting (and I know I do this sometimes too) it’s because the are ‘standing’ in their stirrups. They aren’t standing, but they aren’t sitting either, and they aren’t in 3 point or 2 point. I find that I do this when I’m trying to slow down a horse that isn’t meant to be going the speed of which it’s going (eg. if he / her spooks or bolts). So if you’re doing that, try to focus on siting not standing.
Another way that might help is if someone films you, so you can see how you and your horse are when you ride. If you’re in a lesson, you can ask your coach if you can have a go and then look at the video to see what you need to fix. Or, if your coach doesn’t let you, or you don’t want to waste lesson time looking at videos, you can complete your ride and look at the videos at the end.

I really hope this helped!! 😊😊
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