A trainer had me canter 3 strides 2 point, 3 strides sitting, 3 strides 2 point, 3 strides sitting-on & on. It really helped me & my strength. We worked on it a lot until I could do it a mile each direction without getting exhausted & winded.
Use a spare piece of leather around their neck as a jump strap and hold onto that. Also work or your 2 point when your horse is trotting and cantering without jumps - you will have a much sturdier position over jumps. Also keep your heels down and eyes up - with will open up your chest so you don't dive down
Exactly as Abigail said. Don't be afraid to grab the mane, even professional riders grab the mane sometimes. It could also help you if you play around with going into your two point sooner before the jump.
It sounds like you're being left behind while jumping, practice grabbing the mane at the base of the jump and make sure to keep your leg on tight all the way to the jump. Also make sure not to pull on their mouth while in the air!