The enemy within the gates
This enemy within the gates is the tendency, which we all have, to dwell on the negative, to wallow in gloom and doom. Everyone's situation includes both good and bad. Even if you were the most fortunate person on Earth, it would be quite possible for you to make yourself utterly miserable if you insisted on contemplating only the unpleasant aspects of your situation (for there would still be some) while dismissing or ignoring the positive.
The reason this matters is that the way we perceive our situation can affect how we deal with our situation. Wallowing in doom and gloom leads to pessimism. Pessimism leads to despair. Despair leads to paralysis. And paralysis leads to defeat even when defeat was not inevitable. If you convince yourself that everything is hopeless, you will not take action, because you will have trapped yourself in a state of mind where you believe that taking action is futile or will even be counterproductive, regardless of the actual track record of such action in the real world. This is why the enemy within the mental gates is so dangerous.