FWIW, you ask if you're being a horrible jerk for not wanting to go; I think it is your friend who is being self-centered and insensitive. Who's to say that people would choose to spend their hard-earned vacation dollars on a cruise, especially one centered around a wedding, possibly surrounded by the very people you need a vacation from. Not to mention, as you say, using up all your husband's vacation time. As your friend says, those who can come will and those who can't won't. Just kindly tell her that you are sorry but you won't be able to attend.
eta: oops, I hadn't read anything but your first post when I posted. Congrats on telling her! I'm sure it feels like a weight has been lifted from your shoulders! You did the right thing....