What is mentally different about sailing single-handed?

Nobody contradicts you. In a crew the navigator corrects the skipper, the bow says the sea is bigger than expected, and somebody notices a frantic tone. Alone, every misjudgement runs through to execution unchecked. You are the decision maker and the check on that decision in one person, and you are that on a sleep deficit.

Then there is the second thing: fear cannot be shared. In a crew, another person’s glance already takes something off its size. Alone, it grows into whatever room you leave it.