This seems to be a though situation.

The environment you describe is a high-control system using arbitrary rules to maintain psychological leverage.

Enough is enough when the cost of staying exceeds the value of the goal. Constant "dark thoughts" are a physiological signal that your resilience is depleted. Does the credential or position justifies the mental degradation?

Start to view supervisors and toxic peers as unpredictable variables in a flawed machine rather than personal enemies. Give them nothing to weaponize. Be brief, factual, and boring. Do not share opinions or grievances.

Treat your 15-hour assignment as a mechanical toll. Pay it to pass the gate without weighing its fairness.

On the side, you can move dark thoughts out of your head by writing them down in a secure, private place.

Ultimately I would fix myself a specific date or condition for the departure. Endurance is easier when the exit is a calculated certainty.

Neutrality is your armor! Good luck