I want to attend the party after my company's dinner and dance when my girlfriend would like to spend some time with me before I go overseas for a week
That is a shame you are leaving soon. Why wouldn't you want to spend time with your girlfriend?
I'm not really sure, I just don't want to think about leaving.
I'm sorry you are in this situation. It's wrong to let down a significant other when they could really use you. They are going to miss you when your gone so it's understandable they just want some time with you.
I can't help it, it's just how I feel.
It is expected that you be there for your significant other. Maybe you can change your schedule and find some more time for her right now.