Practicing new tricks is mentally tiring for your dog. Keep it fun and end the session while your dog is still wanting more. End on a successful note, even if you have to go back to an easier behavior to achieve this.
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion.