Вы понимаете данную фразу(написать на ) it is not the mountain we conquer but ourselves. (не горы мы покоряем-самих себя)

мы покоряем не горы , а самих себя

I understand it so: "Internal fight of the person with itself is much more terrible and more dangerous than fight against elements. After all not everyone is capable to subdue itself(himself)".