To answer this question, you might look more closely at Willy's own past. His father deserted his family when Willy was three. The flute music that sounds throughout the play in Willy's flashbacks is the reminder of the father that Willy never had. Consequently, Willy doubts his ability to be a good father. In one flashback, he seeks Ben's approval about the way he is raising his sons:
Oh, Ben, that's good to hear! Because sometimes I'm afraid that I'm not teaching them the right kind of--Ben, how should I teach them?
Willy desperately wants to pattern himself and his sons after his father who according to Ben was a "very great and a very wild-hearted man," yet Willy is not certain that he is following the right course either for himself or for his sons. In fact in the same flashback, he begs Ben to stay longer to help him figure out if he is on the right path with his sons and with his career:
Can't you stay a few days You're just what I need, Ben, because I--I have a fine position here, but I --well, Dad left when I was such a baby and I never had a chance to talk to him and I still feel--kind of temporary about myself.
Willy is uncertain that his career in selling would meet his father's approval.
I think that Willy does have a singular focus. He wishes to be a success. It is within this conception of the perceived fulfillment of the American Dream that we see Willy demonstrate misdirection and a lack of focus. Willy's own flashbacks and sense of subjectivity is vastly different from his reality. It might be within here that Miller is making a statement: In our own reminisces and dreams, we are more driven, more successful, more compacted than we are in reality. In his dreams and flashbacks, Willy is a more caring father, far more successful, and a provider who is more self-assured and more of an embodiment of "success." It is in reality, when Willy leaves his subjective perspective, that the challenges present themselves. It is in reality that Willy lacks personal conviction, cannot seem to find a path to make himself or his family happy, and only in reality where things seem to spiral out of control. I think that contrasting the flashbacks and his own conception of self with who Willy actually is outside of these domains might be a good starting point in developing the idea that Willy lacks personal conviction and direction.