I'm confused about the meaning (in the anticipated experience sense) of "difficulty". As a question: what observable difference is there between task A being more difficult than task B, versus not?
Obviously, if you wonder about the meaning of a word, the solution lies in the dictionary. Trusting the dictionary too much leads us to conclusions like "bread is a psychoactive drug" and "touch is contact, contact is touch, hwæt?", but we might as well try.
Wiktionary says "The state of being difficult, or hard to do." For "difficult", we get "Hard, not easy, requiring much effort." "Hard" is defined in terms of "difficult" and "effort".
We seem to have reduced the question to "what is effort?".
Effort isn't energy expended. If it is, why do we consider thought-based tasks (which use much less energy) to be just as effortful as manual tasks?
Effort isn't concentration. If it is, why do we consider e.g. sustained running effortful?
Effort isn't pain (contrary to lsusr). If it is, why are some tasks (e.g. research, for some people) both enjoyable/painless, yet effortful?
Difficulty isn't (inversely proportional to) probability of success. If it is, why don't we regard the lottery as "difficult"?
Maybe difficulty is (inversely proportional to) probability of success, but only applicable to skill-based tasks. But probability is in the mind, so the difficulty of a task, for a constant actor, would vary depending on knowledge.
Perhaps "difficulty" is an illusion induced by a flaw in our minds. Further research is needed.