1. If you have that urge to go, it is probably for a good reason, agreed :) Wouldn't call it a fail though if you didn't end up going. We all require balance.
2. Parties are for getting together, costumes are just a dress code. They'd love your company even if you didn't dress up -- that's why they invited you after all. So don't stress over it. You can come in something silly or minimal fuss.
> They'd love your company even if you didn't dress up
This is generally true and reassuring, and but in this case I have to put at least something reasonable together since I half-assed it last time lol. I'll probably just try and attend every third costume party in the future