The main thing you need to work on is your lower leg position. Your heel should be under your hip. I had one trainer explain it as if your horse was taken out of the picture, would you be standing, or would you fall down? I'd suggest working without stirrups at all three gaits. Not that much fun, but it really is the best way to develop a good lower leg position. Your lower leg is your base of support, so you want it to be really solid especially if you are learning to jump. IMO, you shouldn't learn to jump if you can't ride at all 3 gaits and in two point without stirrups.