Oh, yeah, for the record I don't think being bitter about the ex is necessarily what makes her a miserable person by itself, and it's 100% valid to have emotional issues with being in a wedding where your perfidious ex is going to be everywhere, but there's plenty of other hints in the letter that she's probably not fun to spend time with at the moment - including the very first line where she makes it clear that the concept of "generosity" is completely foreign to her.