From a recent article entitled "22 Things Happy People do Differently"

9. Get absorbed into the present.

Happy people don’t dwell on the past or worry about the future. They savor the present. They let themselves get immersed in whatever they’re doing at the moment. Stop and smell the roses.
I get being a little bummed that it's going to end at some point, but if it's reaching the point of causing legitimately depressive/sad feelings then I don't think that's an especially healthy way to approach life. Hope you guys have a great trip and are able to focus on enjoying it in the present.