I would suggest you go to the party and decide to try make it a special memory to cherish.
Recently i lost my best friend and it really helped me to keep reminding myself not to be sad because it's over, but to be happy because the friendship happened.
Technology gives you many ways to stay in contact with her and you could even try plan a future visit before she leaves.
Its hard to let go of a close friend but try see this as one door closing as another door opens for you.
Best wishes