With regard to sailing (and speaking from direct experience), and in response to your first two points: You're used to operating a "machine" that has quite a few things in common with an aircraft (wings, operating in a fluid medium), and you're used to being able to control it very precisely. When the water gets lumpy, and the wind is shifty/gusty, you anticipate how that machine is going to respond, and continue to control it precisely. Now you sit yourself in an aircraft, and beat yourself up because you're not achieving the level of control you're used to. Is it possible that you're unconsciously setting yourself unreasonably high standards?
Rather than thinking of someone learning to sail a large wheel-steered yacht (everyone knows how to operate a steering wheel), think of someone learning to helm a relatively tippy dinghy with a tiller extension. It takes a while even for them to learn which way to move the tiller. Now add in the fact that they have a sheet in the other hand, and moving either sheet or tiller seems to want to make the boat capsize one way or the other. Half the time the boat feels like it wants to capsize even if they don't move either. The only way to feel "in control" is to figure out how to coordinate the tiller and sheet, *and* respond to the constantly changing wind. That's before we've thrown in manoeuvres, collision avoidance etc, and it feels like you have to learn it all at once. Now think how "in control" your student's going to feel after 5 hours in the boat; if they're much over 20 years old, the answer's most likely going to be "not very". IMHO, that's closer to what learning to fly is like. Stick with it, you'll "get" it, and ultimately you'll have just as good a feel for an aircraft as you already do for a boat...