Trading is easy. Trading at a profit is hard, very, very, hard.
Some of the most difficult things have been mentioned above.
But I would also add "frustration" to the list-
The frustration of not being in a position to take advantage of obvious profitable trades while your capital is tied up in open positions on others. (especially when the open position is currently a significant loss)
The 'risk' call then becomes 'is an immediate loss worth the potential gain to be had?' -
not something that most humans, by nature risk-averse, excel in at all.
It's a greater barrier than simply 'deferred reward' (which most fail anyway), it's, well, it's instant pain for a maybe...
Hence the many adages like 'a bird in the hand' & 'the grass is greener'
Thus willpower is no small part of trading a strategy.