practice with little jumps n poles on the floor, then get higher and higher, if this don't work go see a riding instructor
Also work with poles on the ground and have someone watch you and make sure your position is not too forward or backwards and just pretend that the pole on the ground is a normal size fence
Improve your two point seat, keep your wide angle view, do not look down! Weight in the stirrups (for balance), stay close to your horse and stretch your back when you are above the jump. Keep your elbows in and your hands forward, allowing the horse to stretch his neck.
Canter and trot in two point