Please read how the neck works before reading the following.
The effect of gravity on the head is that it moves down and forwards, away from the body. As a result of the head being lowered it then has to be rotated upwards in order to look straight forwards not down. This puts a combination of shearing and compression forces through the structures of the neck causing damage, pain and, when severe, other symptoms like weakness. This is worsened by the strain of training during which the head often moves even further forward and is tilted up. After years of holding the head in the incorrect position it is often impossible to correct it without help and self-correcting can make it worse, so you must see a physiotherapist who can take you through the posture correction below.