How do I hold up under demanding, exhausting conditions at sea?

You have the skills now. The sea has stopped testing whether you know what to do and started testing whether you can keep doing it when you are cold, wet, and worn to the bone. That is a different kind of trial. Enduring hard conditions is not about knowledge anymore. It is about pacing yourself so you last. Exhaustion at sea is dangerous precisely because it tempts you to cut the corners your training says not to cut. The answer is not to push harder through it. It is to protect your reserves: sleep when you can, eat when you can, and do the small maintenance on yourself the way you would on the vessel. A rested body makes fewer mistakes than a heroic one running on empty. When the conditions are grinding you down, focus only on the next watch, the next task, not the whole stretch ahead. The sea rewards the steady, not the frantic. You already have the seamanship. What carries you through the exhausting runs is not more skill but the discipline to guard your own strength, so it is still there when the sea demands everything you have.