YES! no stirrups! Is a must, also I think bare back riding has helped me sooo much in all parts of my riding. Just take a few weeks and don't take your saddle out!
No stirrups flat works help a lot
Instead of gripping with your knee (which will cause your leg to swing back) open your knees and hip. Make sure that you are using your hips as a hinge and push your heels down.
You might be gripping with your knees, I grip with my lower leg (and not my knees) and this keeps my heels down and legs forward. I find when I grip with my knees my leg naturally swings back which lifts up my heel. To get my lower leg on and knees off, I turn my toes our slightly (yes I know some people hate this but no one has ever told me not to do it) and push my heels down and forward, practice this on the flat in 2 point and 3 and then try over a jump.
Just keep reminding yourself to keep them down. But. You can do a lot of no stirrups over jumps to train your leg aswell
Practice flatwork with no stirrups. Walking, sit trotting, rising trot and canter. then try jumping without stirrups and see how you go