I understand what you mean, there's no good way to do it, is there? But better now than letting it drag out, I think.
I think telling her she is still welcome on the trip is absolutely the right thing to do. It could still be a great trip, even if not a romantic one. If you have hung out together for this long, it would be nice to think that you might be able to be friends in the end. Hard to actually pull off, but a nice goal.
