I don't want to go to my best friend's surprise party
Why, would you just prefer to stay home?
Yes, but I've been planning a surprise party for him for a while and I think I'd let him down if I didn't show up.
I can definitely relate to not wanting to go out and be social, but since you've planned this party for your friend you should make an effort to go. You don't have to stay all night, but you should go to support and celebrate your friend.
I'm not sure if I can...
You can do it! You should at least show up for a bit! I do not think that it is okay to skip your friends birthday party.