Willy is referring to his uncertainty about the future and his inability to conceive of life without a father. He doesn't know what his life will be like, especially without a father to guide him. Temporary in this quotation suggests that Willy lacks a sense of stability or foundation as if he, too, will disappear as his father did. If not Willy himself, then will other members of his family, such as Ben, desert him as well? Willy is recalling the feeling that he didn't know what the future would hold for him.
He and his family had lived a transient life so he had no clear of "home"; therefore, he lacked the security of that kind of stability. Now that his father was gone, he must have felt that the force holding his family together had distintegrated. What would happen to Willy? Was he now responsible for himself? This feeling of temporary implies as well that Willy hoped this sense would not last long, but notice he qualifies his comment by saying it was "kind of temporary." The instability we see in the adult Willy is foreshadowed by this remark.
Temporary means not permanent. This line provides insight into Willy Loman's character. It shows that the early abandonment he experienced when his father deserted the family has influenced his psyche even into old age.
When Willy admits he feels temporary about himself, even at this late stage of life as he nears retirement age, he reveals that he has never developed a firm sense of identity. In his early years, he hung his identity on a vision of becoming a great salesman—someone who at 84 years old could pick up a phone and effortlessly close a sale; someone who would have friends far and wide; who would be respected and appreciated. He didn't realize that such a reputation is earned by depth of character. Willy borrowed or stole that identity from a real-life person he'd heard of once. No wonder it feels temporary. It's like trying on a persona for a while until you have a chance to build your own.
But Willy never built his own persona. Instead, when he didn't succeed, he lied to Linda about his accomplishments and earnings. When Biff was temporarily successful as a student athlete, Willy tried to appropriate that as his identity. He still sleeps with Biff's sports trophies—markers of temporary achievement—above his bed. He had an extramarital affair—a temporary relationship—to stroke his ego, to make him feel like he was somebody. Even the front steps of his home were built with lumber stolen from a nearby construction site, so they can never be permanently his. Biff states that "we never told the truth for ten minutes in this house." It is impossible to have a permanent identity when you're never being honest with yourself about who you really are.
In the Requiem, Biff states, "Charley, the man didn't know who he was." That's what Willy acknowledged in his imagined conversation with Ben: that he felt temporary because he had no sense of his true identity. That struggle with identity stemmed from being abandoned by his father. The ray of hope in the play is when Biff, at the end, breaks free from the "temporary" persona he has lived. He is able to say in the closing scene, "I know who I am, kid."