The frustration is whether to engage in the world or engage in his art. For any type of artist, including writers, the demands and realities of the real world can often get in the way of success in art. The decision is whether to live in your own world or everyone else's.
Paul himself never understands why the master discouraged him from marrying Miss Fancourt. No doubt the master was telling him the truth about how marriage could interfere with his dedication to his writing, but it would appear that the master had written himself out and didn't feel that he personally could be adversely affected by marriage, even though he had been dissatisfied in his first marriage. Paul can never know what his life might have been like if he had made a different decision, but it troubles him to imagine the master enjoying the life he might have had himself with Miss Fancourt. James deliberately raised these questions and left them unanswered.
So, did the master wanted paul not to marry miss fancourt because he wanted her (before his wifes death) or he wanted to protect paul from repeating his mistake? i think its so confusing, im finding alot of difficulties while analysing it.