The problem is not likely going to be the neck (unless a physical problem is causing a restriction), but the entire way of going. The horse's power should come up through the hind limbs, over the back and into the head. When this is correct, the rest of the horse will be. Otherwise, you are creating a false outline where the neck bends in the wrong place and the back is hollow. It looks good to an untrained eye, but is causing a whole load of issues in reality. Begin researching the scales of training.
Do sitting trot and bring your back hands down towards your knees and bring them out wide.