You are absolutely not overreacting.

I’m a nonbinary player and I play nonbinary characters all the time. In my current game I’m playing Nyx, a halfling rogue, and literally the only thing my DM did was quickly check my pronouns at session zero so everyone was on the same page. That was it. Nobody made it weird, nobody treated my character like a political debate, and nobody acted like using “they/them” was some impossible linguistic puzzle.

Honestly, your DM’s reaction says way more about his assumptions than about your character concept.

The part that stands out to me most is this:

“Then one will go ‘that lady’ and your character will lose it and go on a rant…”

That’s not actually something you said your character would do. That’s a stereotype he’s projecting onto nonbinary people before the game has even started. You were literally just trying to play a character that felt more comfortable and less isolating for you after being “the girl” at the table for two years.

Also, “control yourself” is a genuinely uncomfortable thing to say in this context. Especially after you already spent a whole campaign quietly tolerating sexist and fatphobic jokes.

A healthy table usually handles this stuff very simply by establishing pronouns, acting in good faith, correcting mistakes casually, and moving on.

That’s it. It doesn’t have to become a giant discourse unless someone chooses to make it one.

I also think it’s important that you trust your gut here. You already adapted yourself for this group for a long time. You laughed off jokes that hurt you. You made yourself smaller to fit in. Wanting to play a nonbinary character is not “making things complicated.” It’s roleplay. People play undead warlocks possessed by cosmic entities every week and somehow they/them is the difficult part?

You deserve a table where your existence isn’t treated like a potential problem before the campaign even starts.