On the last page This Is How You Lose Her, the finale of "The Cheater's Guide to Love," we finally see a change in Yunior. He flips through The Doomsday Book, his nickname for a folder sent to him by his ex-fiancée. He's kept it "hidden under [his] bed," away from the reader, and from himself; it's the first time we learn about it. The folder contains "copies of all the e-mails and photos from the cheating days, the ones the ex-found and compiled and mailed to you a month after she ended it."
He goes through the whole archive, twice. And he admits that he is cowardly and scared and small. He admits he has lied and hurt. He admits—for the first time in the book, even though "it kills" him—that his ex was right to go.
This is the starting point, this is the baseline. Yunior begins then, as he has tried and failed for months, to write. He scrawls out a terse confession:
The half-life of love is forever.
The women he has loved and lost are in him eternally, like radiation; their cast shadows will only grow, like cancer. And he is sorry.