1. A rider lacking confidence about jumping will naturally lean back and go defensive as they approach a fence. Leaning back, straightening the elbows and raising the hands will tighten the rider and immediately transmit a negative message to your horse. The horse will quickly realise you are not sure and possibly refuse or run out. These bad habits need to be resolved, to achieve this you will have to take a few steps back in your training and learn to become totally at one with your horse's momentum. Be honest with your trainer, they will be able to help and advise you on the progression in jump height or/and type until you are confident.
2. A loss of balance may also be an issue, work on your seat position and if possible have a lesson on a lunge without your stirrups, this will make you sit deeper and feel the movement of your horses' stride. Your personal fitness will also help, strengthen your back and core muscles giving you better self posture and support for your body when the horse is in flight and out of the saddle.
4. Practice jumping over small obstacles or through a small grid with no rein contact. If you need to hold on to something use a thin stirrup leather around the neck or the martingale neck strap. The removal of the rein contact will improve your balance and the more you relax the more feeling you will gain for the horse.
The above exercises are also useful for the opposite problem of throwing your body forward in mid air. Gripping too rigidly with the lower leg and knee will result in the rider having to throw themselves forwards to maintain the balance. Remember not to try and jump the fence for your horse by pushing up on take off, this is ahead of the movement and will upset the horses' point of balance.