Why does Okeke have trouble sleeping at the end of the story?

Okeke has trouble sleeping at
the end of the story because he's in a state of emotional turmoil:

"That night he hardly slept, from remorseand a vague fear that
he might die without making it up to them."

He has
just received the letter from his daughter-in-law, Nene, in which she begged Okeke to
reestablish his relationship with his son, Nnaemeka (Nene's husband), and to meet Okeke's two
grandsons for the first time.

Okeke has spent the past eight years in
bitterness, refusing to see Nnaemeka, because of his marriage that Okeke doesn't approve of.
Now, Okeke is a grandfather who has never met his grandchildren. Despite the cruel words and
harsh silence Okeke has always extended to Nene, she has reached out to him with warmth,
kindness, and respect, via the letter, and offered him one last chance to connect with Nnaemeka
and the young boys.

So you can understand how Okeke feels
"the resolution he had built up over so many years falling in," meaning that his harsh
feelings toward his son are starting to crumble. He tries to "steel his heart" and
avoid thinking about the grandchildren, and he tries to distract himself by humming a religious
hymn, but he can't help visualizing the grandchildren and thinking about how he has essentially
abandoned them since birth.

In sum, guilt, worry, and the sad
realization that he must change are the feelings that are keeping Okeke up that
night.