From one nonbinary person to another, hello! I'm sorry you've been having such a hard time of it (in ways many of us so often do). Also, I love your signature line tremendously.
I would suggest you trust yourself in terms of what you know about your own identity, rather than worrying about copying someone else or being perceived that way. Honestly, even if there was some of that in here, so much of how everyone performs, understands or talks about gender is based in mimicry in the first place: even our idea there is such a thing as gender is inherited, you know?
The bias about being nonbinary you're talking about -- about it being a phase -- is very similar to the one like that about bisexuality, and they're both bollocks. The idea that only poles of a binary system, and binary systems, period, can be fixed places, and that any kind of fluidity is just about movement towards them is just that: an idea. And a wrong-headed one at that. You know from experiencing this directly: that's not how this works.
I'm sorry the other nonbinary person at school behaved that way with you and responded that way to your disclosure. That sounded very dismissive to me, and I bet that experience felt isolating and crappy.
Let's see about troubleshooting this for you: what do you want -- what are you looking for -- in coming out? And can you think of anyone you might try coming out to first who you can for sure or probably count on to be acceptive, supportive and to validate your sense of you identity instead of looking to invalidate or dismiss it?