Learning in aviation is a steep learning curve: PPL, IR, ATPL, type rating etc. Confidence comes with time and as to forgetfulness, hands up if you've ever accidentally pulled the mixture instead of carb heat.
AD you have a valid point maybe in a different way to intended. Even experienced pilots thrown into an unfamiliar aircraft are all fingers and thumbs.
You only really know an aircraft when those fingers flick instinctively around the cockpit while you are thinking about what you are doing at the weekend.
I can remember learning to fly instruments all your concentration was 100% fixed on keeping the thing upright add other tasks like radio work or navigation tasks and everything went pear shaped.
Far off from today where it makes no difference in or out of cloud where your eyes scan instinctively and often flying instruments you can find your mind wandering to other thoughts its now so easy and natural. The other way around and you don't really know the aircraft
the same is the process learning to fly or riding a bike its only when it all comes instinctive and takes no mental power to do it can you say you have a full grasp on flying.
Its the same with checklists I think it was the RAF who taught you to fly without a checklist before you were ready to use a checklist if you get my gist )
The one means you really know the aircraft the other means you are flying by numbers )) The one the checklist is a backup the other its a command list for a lack of knowledge and familiarity. When it becomes a backup you know you have it
Pace