Answer:
But I am beginning now to fear that I must wait a lifetime.
Step-by-step explanation:
This is the sentence that makes the audience notice that the narrator realizes that the Time Traveller might never return. In the previous sentences he is only waiting for his appereance, although he felt a strange feeling, he didn't show any despair until this sentence. The word "fear" is the one that gives us an important clue, this is the moment he assumes the worst.
By staying there an waiting for the Time Traveller, the narrator shows that there is still hope of his return. But by stating that he might wait a "lifetime" he also shows that this hope is almost imperceptible.