Just chiming in to agree with everyone that there is no substitute for just doing two point. If you can borrow the lesson CC saddle, or get your hands on a cheap one that fits you and your horse(s) well enough to do no harm, do walk and trot sets in two point until you feel stronger and less achy. Then do canter work in two point. Then go find a good place to gallop, bridge your reins, and do that in two point. Think about a line flowing from the top of your head, through your body, down your legs, and into your heels, letting your hip open and close as needed with the movement of the horse, not pinching with your knees, etc. so you are sinking into your heels but using your core to keep that line taut.
You don’t need to shorten your stirrups to jockey length, but several holes up for jumping length is going to get you where you want to be. I do dressage and jumping in my CC and go up five-six holes when I jump from when I flat. I can trot all day in two point because I also ride an airbike 5-6 days a week, but holy hell it hurts the next day (and the next!) unless I do two point regularly!