Gun shy.. or gun nervous? the two are different. if its gun shy then not much will correct it and 12 months to correct a working dog is impractical in my opinion.
Gun nervousness can be cured but stop using the shotgun, start with a starting pistol or similar with the help of an assistant, using open ground start 300-400 yards away and let off a shot if any reaction at all treat and reassure once there is little or no reaction slowly decrease the distance.. then start again at distance with the shotgun and repeat the process, It is difficult to asses which it is but gun shyness in inherent