I would say your translation 최악의 적은 너의 신경계이다 is correct.
However, we more commonly hear 너의 제일 큰 적은 (바로) 너 자신이다 (Your greatest enemy is your own self). "nervous system" seems to be narrowing it unnecessarily, as our limitations are not only in the nervous system but in other parts as well.
너 자신을 구하라 sounds more accurate for "Save yourself". 지키다 means to defend (방어하다) or to protect (보호하다). 구하다 means to save or to deliver (from the enemy, evil, etc).