Scarleteen

Okay. Based on my gender expression and how I've been feeling, I think I might not be cis. The closest term for myself I can think of is demigirl, but I'm not sure. Also, if I actually turn out to be cis, I will feel bad because I'll feel like I've taken away some of not fitting into the binary from people who don't. I know, I worry about a lot of really stupid things

How you identify yourself doesn't take anything away from anyone else, I promise. Your identity isn't about anyone else, it's only about you. As well, all of our identities not only can change or shift throughout life, they often do at least shift, because who we are at, say 15, is rarely who we are at 30, 45 or 80. So, in the event someone's identity now isn't what it is later, that, too, is okay and doesn't take anything away from anyone.

So, how do YOU want to identify right now? What feels best to you for you, based on what you know about yourself so far and who you feel like today?

I think cis works for me right now, though I do like demigirl

If that's what feels like it fits best for you right now, that's totally fine. As Heather said, identity can shift over time so if you feel like a different word or set of words works better in the future, you are able to update your identity at any time. There's no limit on how often you can settle on an identity or terminology that fits you best!