You just answered part of your own question! Collecting and lengthening your horse's stride on the flat will translate to your jumping, as well when you need to adjust for a distance. To gain control of your horse's ribcage and to get them listening, you can work on a counter bend to a correct bend. For tighter turns, make sure you have your horse moving off of your outside leg. Something that really helped me was to set up a 'box' with groundpoles as the sides. Make it about the size of a 20m circle. Walk, trot, and if you want canter on the inside of the poles and focus on square turns, even a turn on the haunches in each corner. If you want, you can also try some transitions- trot down the side, walk and turn on the haunches in the corner, trot down the side, turn on the haunches, etc. Serpentines help, as well. Any kind of lateral work will help you- use the leg-yield, shoulder-in, haunches in, etc. to gain control of your horse's entire body and get both him and you more in-tune with each other.