When Thoreau said "To thine own self be true" I bet he didn't realize how widespread a mantra it would become. The self-esteem movement of the last few decades has certainly changed the way we see ourselves, and created a broader sense of personal freedom in our culture - but not without some confusion.
I've always taken issue with the notion that "you can be anything you want to be if you just put your mind to it." It's a nice idea on the surface, but it's rooted in the mindset that you must become something else to be whole and fulfilled as a person, and betrays the very value it is supposed to promote. If we base our self-esteem on a future situation, we must always be waiting to feel whole. This makes it impossible to find inner peace in the present moment which is the only place that peace can be felt.
That kind of flawed strategy comes from an egocentric mind held hostage by its addiction to thinking and striving and solving hypothetical problems that aren't even real. We can easily get swept away into mindlessness while trying to construct a healthy self image and never recognize that a self image is still just imaginary.
But Thoreau wasn't talking about a fabricated self image anyway. The "self" he was talking about is the Universal self that transcends our egocentric mind. The mind is only a part of the whole being just as the body is only a part. Being controlled by your mind is like having your legs decide to take over and start telling the rest of you what to do. It's silly, but that's exactly what our minds are doing. Most of us don't realize our egocentric minds have run amok and are continuously causing all sorts of problems.
Our "true self" is complete and whole in every moment and doesn't need to "do" anything except remain still and know itself without judgement. With this awareness we are finally free to exist and act with authentic integrity towards ourselves and others.
Ironically, the most self-less thing you can do often feels the most selfish - nurturing your true Universal self.