The hardest part for me has been learning to relax. As much as I enjoy it I find it to be stressful more often than not. A good stress but stressful nonetheless.
You may have hit the nail on the head. Keeping calm, and thinking things through, is something that many seem to have a problem with.
That's similar to another lesson that people have a hard time with. Based on my observation, it appears some people operating the same boat don't mix.
It never ceases to amaze me that we were able to hook up to a mooring ball, with no verbal interaction (two hand signals only), the first week we sailed together, and others do it for years, and still yell and swear at each other.
My wife and I still joke about a couple we encountered at a dock in Desolation Sound. When in skinny water, either she or I says exactly what we heard when a couple was docking and yelling at each other..... "There's rocks here !!!!"
So far, the only time we have yelled at each other, is when I thought we were screwed, and a strong cross wind was going to push us into a cat. She looked at me with saucer eyes, from the bow holding a roving fender, and I resignedly yelled "I think we are *ucked !!" 2 seconds later I was fiercely determined for one last attempt on getting the bow through the wind as we were being blown down the docks in tight space, on an unfamiliar charter boat. We made it with a few inches to spare.
Maybe it's the headspace that is the hardest thing to learn.