The ancient Greek aphorism "know thyself", is approaching the mental actualization of understanding yourself. Essentially seeing your personality naked, without the obstructions of the conscious mind.
How you might come to understanding yourself, is perhaps the hardest question to answer. More often that not people waste so much time on the how, that they fail to do it. This could be in the spiritual expedition of religion, or perhaps through the more philosophical practice of meditation. One thing I often say is that meditation doesn't have to be related to religion at all, which is often surprising to most people. Alas it is simply a form of mental exercise, with the hope of strengthening the brain/mind's resolve.
Use it as a bridge for the conscious mind, and subconscious.
The ideas and thoughts you contain within can actually greatly impact your world on the outside. Your body is the physical manifestation of your mind. As out there as that sounds it is true, and if you fail to condition your mind your body will follow suit.
Train the brain to relax, and your heart will relax. Train your brain to push past breaking points, and your muscles will listen (literally was the only reason why I was able to do 300 push ups one day). Train your brain to control your breath, and you can hold it for much longer or resist cold.
Your thoughts create your actions in the real world as well. Never forget that.
Your emotional state has a profound effect on your temperature of your body. Akin to the last section about how your mind-body connection is important; The emotions are important as well, for there must be awareness of your emotional state in order to make a logical decision.
Many people let the primitive side of their minds dictate their actions, which essentially means they are run by there emotions. The primal instincts have evolved for a reason, and the higher mind is key to understanding yourself. As well as understanding others through empathy.
Your microbiome of your stomach, the synapses of your brain, the twitches of the muscles (fast and slow), and the breath regulatory system; All of them are areas that directly effect your daily life, but most people fail to learn about them. Let alone actively study them, for the sake of improving them.
Your actions are determined by your thoughts, and your thoughts can only originate from what you know. If you do not choose to learn, in the effort of maximizing the efficiency of your internal operations, then you are subjecting yourself to a subpar existence.
No one can be blamed besides yourself.
All of this comes down to knowing yourself, and who you are. More over knowing who are over time, and who you can become. The future version of you is simply who you plan yourself to be by the actions you take now. If you see yourself as a fit person, but you are not now; Then you must take the actions that will drive that existence to come into being. Such as exercise and nutrition, then your physical world will manifest your internal vision of who you are.
If you know yourself to be something, but others do not see it then you must change your actions. Your actions are simply the choices you make on a daily basis. If you know that something is to define you, but others do not see that then you are not taking the actions of your own definition.
If you do not know who you are now, or who are to become then you must search within. Perhaps even experiment over time, and discover that about you. The sooner the better! :)