I want to enjoy this trip but how do I do this knowing she is with someone else and I have been so discarded?
By using it. Travel is a great way to break us out of our routines and provide new stimulus, or at least distraction. You can make a decision that you're going to have a great time, but then you'll compare the actual time you have to that. Instead you might use the hatred you have for her as fuel to insist you're going to dive into whatever shows up while you're away all the way, and use some anger to discard her and insist you have some fun with whomever you meet despite her. This was a vacation the two of you planned together, so why not use it as a tool to get over her? Only seems fair.