...if P&P included an example of a lady who never did get married, which would have been an example for the reader to see Charlotte's consideration of what it is like to never marry versus what it is like to marry. We have plenty of examples of the latter.
Charlotte has the intellect of Lizzy, but not the beauty or the charm. Jane Austen, in her letters, seemed a bit more like an Elizabeth Bennet, with her wit and personality. IIRC, she did accept someone's hand and then later rejected it, correct? For Charlotte, we don't know her background. We don't know if she was an Anne Eliot who refused a man, having been given advice that it would not be a good match, only to be so happy at the second chance. Charlotte it seems was never beautiful on the outside, perhaps with no opportunity of being asked before. Or maybe she was asked and she turned it down, and now has spent a few years contemplating the alternative of living in her situation indefinitely.
I recently was listening to a course on tape about Colonization in America... the professor alluded to the fact that in the 18th century in England, about 10% of women remained unmarried. I don't know whether to judge that high or not. Or how that compares with today (maybe I'll look it up.) But it's clear from the reaction of her brothers that they are happy she won't be a burden on them, her sisters are happy because it sounds like unlike the Bennet family, the younger sisters had to wait until Charlotte got married to be introduced or to be "out". She probably relieved a burden on her family that lifted everyone's spirits. She may yet have moments of terror regarding her impending marriage after it has taken place. But the mind also has ways to mitigate unpleasantness until there is a way of perhaps lessening it or doing away with it entirely (perhaps Mr. Collins dies a mysterious death somehow or other shortly after 1) Mr. Bennet dies and 2) Charlotte and Mr. C have a son. ;))
One other thought, just like any other endeavor, if your GOAL (just like Mrs. B's is to marry off all her daughters) in life is to get married, I can perfectly understand Charlotte's willingness to jump off this cliff, even if it risks severing the affections of a dear friend such as Elizabeth.