If you want to overcome the voice in your head that tells you "you can't", you must find the voice inside your heart
Every one of us must battle against ourselves, against that voice in your head that tells you to stop, you are too tired, you can't do it. But you can. There is always more left in you. You just have to learn how to access that power. To learn to access that reservoir of strength deep inside your soul, you must overcome the fear of pain. Fear is what keeps you from pushing yourself past that limit. But you have the ability to overcome fear. Will is the greatest power in the universe (k, I stole that from Green Lantern, which is a great movie btw).
If you can master your will, you can master control of your mind. It is one of the hardest things to do. I know. You have to learn to trust in your training and trust in your body.
When you can let everything go in a race and kind of 'tune out' and just run that is when you will have your best races. Stop over-thinking. When you analyze every single thought and every single move, you are thinking way too much. Stop thinking about how much your body hurts. Stop thinking about how many people have passed you. Stop thinking about if your hair looks cute (admit it, you've had this thought in a race before). Instead, focus on the moment. Focus on your breathing. Focus on how strong you are. Focus on how exhilarating each moment is. Focus on your strengths.
Push that voice inside your head aside and be the stronger person. Overcome the doubt and fear you have. It takes practice and it takes courage. Its something you must practice on every single training run.
Courage isn't being fearless, it is having the strength to overcome fear
How do you overcome that voice in your head?