Your worst enemy cannot harm you as much as your own unguarded thoughts.

Other people can hurt you, but they are only present some of the time. The stories your own mind tells, repeated without examination, go everywhere with you: that you are not good enough, that everyone is against you, that nothing will change. Left unchecked, those can shape your choices and mood more deeply than any outside opponent.

This is not blame, since nobody chooses the thoughts that arrive, and for some people they are loud because of illness or hard experience that deserves proper help. Guarding here simply means noticing. When a harsh thought repeats, ask whether you would accept it from someone else, and if not, question it.