Your upper body is likely getting left behind so your legs go forward. It’s a core strength issue and not using that strength to control your hip angle. Without it, you can’t get your weight properly in your heels, a flexed heel does not mean proper weight distribution, often means pushing off the toes which does not stabilize the lower leg despite the low heel position.
Without video impossible to say for sure but my trainers would pull the stirrups off the saddle and lunge me on the flat at the canter to start, then over tiny jumps, then off the line down a simple gymnastic…over time of course, not a single lesson. If you do your homework it should go pretty fast. Homework means lose the irons. Force yourself. Walk if you have to, takes awhile to build the strength to just let your body follow the motion of the horse and not fight it.
You might also try closing your eyes a stride out, Probably trying to force your body over instead of just letting the jump fold your hip angle and not moving anything else.It helps most people who overthknk everything.
Pilates and yoga seem to help isolate the core muscles and don’t require any coaches, equipment or speshul shoes. Do it anywhere, just work it in when you can,