Meep: I know it might suck, or seem like you don't have anything cool. Look at it this way: my first con was Kumori 2003, I wore a straight out of the closet Chii, I had thigh high boots, and hardly anyone knew who I was except for people who'd seen the anime and knew who I was by my ears.
I was hot, miserable, left alone 90% of the time, but everyone who saw me, commented on how cute/hot I was. It really made my experience so much better than I'd imagined~ I think that even if you have one costume, you can still rock it and when people see you in that, they get excited, want to hug/glomp you. It's just really rewarding. I really think you should go.
If you're worried about having a group to go with, you're more than welcome to join me <3