Bareback is best for everything!But if you can't, I like to do serpentines stirrupless for balance and gridwork ( low then work up to high) for leg positioning but always have someone watch you during gridwork to make sure you maintain a proper position.
I tend to go bareback and get someone to lunge you and do rising trot either with or without your horses mane depending on ur balance and I also do it in canter.
I would recommend you to avoid taking up the jumping position too early, or too late, learn to count your strides into each fence, so you are able to go with your horse at the right time, every time! :)
Here's an article that you can read which is really interesting about your problems and what you want to get:
I hope it will help you :)