Go into 2 point a couple of strides before the fence and grab some mane. The 2 point will help get your heels down and grabbing the mane will stop you from hitting the horse in the mouth when you land. Good luck!!
Also, try to focus on your balance and sit up straight (not falling forward). If you keep you weight in the front you're going to have a hard time to keep your balance and the horse may start to slow down or stop after the jumps. So usually you have this problem when your horse is not forward (new to jumping) or the rider has to work on his/her position in the saddle, hope it helps:)