“Watch your thoughts for they become words. Watch your words for they become actions. Watch your actions for they become…habits. Watch your habits, for they become your character. And watch your character, for it becomes your destiny! What we think we become.”
-- Margaret Thatcher
We are not our thoughts, but if we don't take care to change them, we may become what we think.
That is 100% true! With repetition comes normalcy. With normalcy and habit come manifestations. So you're right we must take care of our thoughts.