Seen a few people think it’s unrealistic that Picard, a very confident positive person, would have become isolated and depressed. First, under the confidence is a man who is quite solitary. While he opens up more in the films, he doesn’t have a family & he keeps his distance...
...from his crew to maintain professionalism. He makes a poor gambit and leaves starfleet, the only thing he has ever known. He never wanted to leave and he feels, rightly or wrongly, that starfleet has abandoned its principles. He returns to the one other place that means...
...anything to him, which isn’t much, his vineyard. Over time becomes more and more isolated and remorseful over what happened and the loss of his friend which is a very human thing to do. Once you get stuck in a rut it’s hard to get out of it. Plus he is no longer a young man.
People are complex, emotions are complex, and making a massive change to your life, however determined and confident they may have once been before, can have a detrimental emotional effect. We are not set in stone, and our lives are as much our surroundings as our personality.