The following quote is from Mansfield Park by Jane Austen:
Mr. Crawford, approaching Julia, said: "I hope I am not to lose my companion, unless she is afraid of the evening air in so exposed a seat." The request had not been foreseen, but was very graciously received, and Julia's day was likely to end almost as well as it began. Miss Bertram had made up her mind to something different, and was a little disappointed; but her conviction of being really the one preferred comforted her under it, and enabled her to receive Mr. Rushworth's parting attentions as she ought.
My question is: what is the referent of it in under it? What does under it mean?