Shooting behind

The best advise is to get a coach. If you can't find one in your area then this is what I can suggest until you find somebody with experience: 

The first cause of shooting behind is stopping the gun before you deliver the shot. This has several causes such as:

Head lifting from the stock. When lifting the head, the visual connection between the gun and the target gets interrupted. The eyes come off the barrel and the sight picture is compromised. Once that happens a sudden gun stopping occurs.

Firing too early, before the gun reaches the proper lead. One of the reasons for a short lead is a bad stance that won't allow full turning to the breaking spot. Setup your stance to have enough room for follow through after the shot.

Measuring the lead occurs when the shooter wants to execute a perfect shot by checking the bead in relationship with the target. By switching the focus from the target to the beat we cut the visual connection with the target thus affecting the gun speed and optimal timing. By looking for the perfect lead (measuring) we compromise the entire shot. Practice to keep your eyes always on the target!

There are other causes to shoot behind the target that are specific to each shooter so the best thing to do is to find a coach that has good professional reputation.