My take is a little cynical, but I think, “I’ve never given my best” means that the poster takes some pride in what his potential best is, and it has the side effect of never challenging the idea of what your best is. Doing good work, and being good at the work are different.
In terms of philosophical perspective that seems to address intrinsic or extrinsic motivations for moral behavior, here being transformed into intrinsic or extrinsic motivations for excellence in professional work, not giving your best may be like simply avoiding difficult situations. That is, a king or general would have more opportunities and complex decisions where they even have an opportunity to be moral, compared to a peasant who has fewer opportunities to be immoral or even make decisions.
All that said, professional work isn’t actually equivalent to moral behavior, and the poster pretty clearly explained that the many other aspects to his life are a much greater priority.