After your horses front hooves touch the ground, say one and then come up this helps a lot and eventually you won't have to think about it.
With my mare, who launches her self over the jump, I tend to sit back as you would normally ride and when coming up the the jump count your strides, as you can't your last stride start to bring yourself out of the saddle with the horse.
This helps me a lot because mine races (and there's no stopping her) I count the last 3 strides before she takes off, it also helps the horse to keep it's balance, if you get left behind or out jump the horse it usually loses its own balance 👍
put a pole in front of the jump about 1 stride away from it, and rise once you feel the horse's back legs go after the pole.
A good way to help that is to stay down in your 2-point for an extra stride and after a while it will start to get better. This is what my trainers do, so hopefully it helps.