I'm not very sociable, so I tend to avoid parties unless I know everyone well. At work, I rarely socialized with coworkers after hours. Occasionally, but rarely.
So I do understand the reluctance to go. But that has nothing to do with how you look.
Even if you don't buy a new dress (though it is a perfect excuse to buy one), you have shown us some really nice ones you already have, and they look great on you.
A survey by Cosmopolitan Magazine years ago found that most women get dressed up to go out not to impress men, but to impress women. However, my experience with "Mandatory" company parties is that some dress to the nines, some dress up a little, and some come as they are and just be themselves.
If you go, I think you should just be yourself. If you feel like wearing makeup, then wear makeup. If you want to wear a nice dress, running shorts, or a fur suit, then do that. The point of the parties is just to socialize, relax, and have some fun. It is not a fashion show.
And if you get there and you aren't feeling the vibe, leave early. Nothing says you can't.
Don't stress out about it. It's supposed to be fun!
