I feel so hurt and alone

I told my best friend I was nonbinary. I did it over text bc we both have busy summer schedules and it's very likely I won't see her in person for a month or two. Maybe not the best decision, but I didn't feel like I could wait to just tell someone. I just told her I was nb, and requested that she call me Simon. I mentioned that any pronouns are fine and said I am still the same person, just a different gender than previously assumed. I felt physically sick and nervous typing it out. An hour later she responded with just "um ok".
She reacted very well when I came out as bi, giving me a hug and supporting me when I told my parents. She's probably just overwhelmed or doesn't know a lot about nb identities, and I'm trying to tell myself this, but I'm still scared as hell that I'll lose my best friend. I'm terrified that she thinks I'm just making it up for attention, or that I don't love her as much anymore, or that I'm not me anymore. I just don't know how to feel about this.
I feel like I'm not a real nb person and being too needy. I feel hurt that "um ok" is the response from my best friend to something that's so incredibly important to me. I wonder if she realizes how much it matters to me that I have someone who knows and how desperate I am for someone, ANYONE to just accept me and love me as Simon, not my birthname. I just needed her to understand and I got such an indifferent answer, like it doesn't even matter. She's the best friend I've ever had and I genuinely think she would never knowingly hurt me, but it still HURTS. She's the person I always by default trust with my feelings. If she of all people makes me feel so insignificant and alone I don't know who wouldn't. I have no one - not my parents, not my friends. Just EC. I hate this.