What was the most difficult skill for you to learn during your training and what did you (or others) do to finally make it click for you?
I'll start. Mine was landings, it just didn't click for me. Flight after flight I'd only have 1 or 2 good landings, the rest were of questionable quality. I finally ended up flying with another instructor at the school and it just clicked. Don't know if it was his insight into what I was doing wrong, but it worked and from that flight forward the good landings far outpaced the bad ones.
Everything, from takeoffs to climbs to turns to descents to landings to traffic patterns to stabilization on final, until I learned the fundamental concepts that “attitude plus power equals performance” and “trim sets the speed the plane seeks while power sets the climb/descent rate at trimmed speed.” I'd heard a lot of the argument between “throttle controls speed while elevator controls altitude” versus “elevator controls speed while throttle controls altitude”, and finally learned those statements were BOTH wrong. It wasn't until I read AFM 51-37 on instrument flying that I learned the real truth, and I'd slogged my way all the way to holding a CFI ticket (how I got that far I'll never know) before I learned that truth and consequently how to control the airplane properly and precisely.
For more on those concepts, see the following:
AFM 11-217 (the current version of 51-37) Chapter 3
During my primary instrument training it was NDB approaches. The instructors kept saying that the needle just points to the station but the procedure for tracking inbound or outbound with wind correction was complicated. It wasn't until I got my instrument rating that I began to visualize what was actually going on with the airplane. None of my instructors just drew pictures of what the procedures were designed to accomplish. Finally, I drew my own pictures and that was the key to understanding it.
After I purchased my Maule MX-7-180 taildragger in 1998, smooth landings became the most difficult skill to master. None of the flight instructors that flew with me were familiar enough with the airplane and its particular configuration to be of any help. I had done my tailwheel transition training with an instructor in a Citabria Aurora and gained skill in both wheel landings and 3-point landings. The Citabria definitely wanted to wheel land more than 3-point. The Maule was a different story. I bounced a lot and had some hard landings. Inevitably, the airplane would float, then suddenly stall a foot above the surface as airspeed decreased and plop down with a jar. I joined the Maule Forum and got tips from other, more experienced Maule pilots. I also found some very good information on backcountrypilots.org about slow, steep landings. A number of things happened to help me master both 3-point and wheel landings in the Maule. I added MicroAerodynamics vortex generators, Alaskan Bushwheels HD landing gear, 31" tundra tires and an Alaskan Bushwheels 3224A tailwheel to the airplane. In 2016 I flew it to McCall Idaho and spent 3 days with McCall Mountain Canyon Flying Seminars. I had flown with them in Lori MacNichol's Super Cub in 2012 but the Maule was a different airplane. They helped me develop a performance card for the Maule in every stall and flap configuration, determine optimum canyon flying speed and flap configuration, perfect normal and emergency canyon turns and back country airstrip approaches, landings and takeoffs. It gave me a chance to perfect slow, steep approaches into the back country airstrips.
Landings are the perennial favorite for new students. They haven't yet accepted the fact that good landings are the cumulative result of what else happened in the traffic pattern. They only want to focus on the last 5 seconds.