Lewis is pointing out that if the object of hope can never be attained as a better state (i.e., it is better to seek with anticipation than to find), then true hope is impossible (with the exclusion of false perception, i.e., "and known to be true"). One cannot hope for a worse state.
One could say that in the glorified state hope is no longer significant. "Now hope that is seen is not hope. For who hopes for what he sees?" (Romans 8:24, ESV) Passing beyond hope does not end joy (but, in this case, makes the joy complete).
With respect to frustration, I am reminded of a comment in G.K. Chesterton's Orthodoxy in a rather different context:
Because children have abounding vitality, because they are in spirit fierce and free, therefore they want things repeated and unchanged. They always say, "Do it again"; and the grown-up person does it again until he is nearly dead. For grown-up people are not strong enough to exult in monotony. But perhaps God is strong enough to exult in monotony. It is possible that God says every morning, "Do it again" to the sun; and every evening, "Do it again" to the moon. It may not be automatic necessity that makes all daisies alike; it may be that God makes every daisy separately, but has never got tired of making them. It may be that He has the eternal appetite of infancy; for we have sinned and grown old, and our Father is younger than we.