Spending hours reflecting on your own life isn't selfish. It's essential, especially right now. You might want to keep things lighthearted for the sake of the social situation, but that just isn't necessary. The people you'll be with will totally understand if you need to be a little bit introspective. You can get wild and crazy some other day when you feel like it. Right now, follow your mood. If it tells you to stick to yourself, then do so.