I agree with Sally, mind games as well as physical ones will help. I have had Brittanys for 35 years since they came to the UK and they are basically a hunting machine on legs, which is fine so long as you are fit and able to take them out shooting or hunting with hawks etc (which I used to do, sometimes twice a week in season
), but as a 74 year old with mobility issues I needed other ways of tiring my lot out. I have bought indoor treat games for my youngster (four this week), and they definitely work as well as the hour and a half free running in woodlands.
They also do 'hide and seek' games in the house, retrieving, and the best game for Missy is the 'chase the hens along the fence and back again' game
Occasionally Henny Penny jumps up onto teh fence and down onto teh grass foor alook see. She chases Missy if she gets too close, and Missy spends a lot of time stalking her around the back garden
But I would add in addition, please do not take her off puppy food too early - pups need the protein and nutrition that most good brands contain.