It comes from Chapter 24 at page 528, and it is from Edward to Bella.

"You, at least, made an effort. You got up in the morning, tried to be normal for Charlie, followed the pattern of your life. When I wasn’t actively tracking, I was… totally useless. I couldn’t be around my family—I couldn’t be around anyone. I’m embarrassed to admit that I more or less curled up into a ball and let the misery have me. It was much more pathetic than hearing voices. And, of course, you know I do that, too."

This quote is of undeniable importance to the closure of the story in New Moon.

When Edward and Bella are reunited, there is still something missing - closure. The strength of Edward's words during the break-up sequence resonate throughout the story, and it seems that there is a need to counteract them with equally profound sentiments such as this one. While Edward's admittance of lying to Bella during the break-up has already been discussed, this fragment of Edward's words go to show his own suffering - a thing that was not seen through the eyes of the protagonist and thusly wasn't not previewed to the reader either.

It goes to show that not only does Edward fathom what Bella went through in his absence - the heartache, loneliness, and the courage that it took to get back onto her feet - but also the struggle that he endured absent her. It reminds the reader that their love is a two-way street, and it also illustrates the respect that Edward has for Bella's resolve and ability to overcome. Hearing of his own loneliness without her brings a bit of comfort and, frankly, relief.