The hardest thing about training has to be mastering one's mind. For me, its the thing that I struggle with the most. Here is a video of Eastern philosopher Alan Watts who eloquently describes the need to develop one's mind.
https://www.youtube.com/watch?feature=player_embedded&list=UUFyAgi9phA7ErVY_bPC1Cjg&v=emHAoQGoQic