To me, it seems like you need more confidence in jumping. You should start with simple balance activities, like Around the world at a walk or trot, or using no stirrups. Bareback work would also help for going over jumps, since it helps with leg muscles. I used to be in the same position as you, so I know how it feels. When you are heading for a jump, you should keep your head up and focus on where you are heading. If you are getting left behind, you should go into a two-point or three-point a few strides before the jump, so once you reach the jump you will already be out of the saddle. Counting strides also helps alot. Counting a simple 1,2,3,4 in your head at a trot will help, or a 1,2,3,4,5,6 for cantering will help you with getting into two-point before the horse takes off.