Possibly just talk to your instructor, if shes ridden your horse she will know and may be able to give you some tips. But shifting your weight and putting your outside leg back will help. Also try changing over poles, it will help get the change without becoming disunited or unbalanced. :)
If you do a figure 8 than once you get to the middle of the shape shift your outside leg back and squeeze or kick. It helps if you have spurs on. Hope it helps😀!!
Shift your weight in your hips and bring your outside leg back kick
Half halt! The change is going to be a million times easier once he’s on his back end! Try doing a small circle in the true lead, go for the change and then a small circle in the new lead. Things that also work would be half passing to the rail and then doing the change to make sure that they’re really sitting. I would also say that when schooling the change, if he starts to speed up and go on the forehand just halt! And maybe even do that a few times before (just going across the diagonal and doing the simple change through the halt) you start to actually school the change! My horse does the same thing so I’ve been doing this for a long time lol