You're in the wrong here.
Your job is not to analyze what someone's gender identity actually is. Your job is to respect the pronouns the other person gave you. Unless they specifically tell you so, you're making assumptions regarding their gender identity. Even if you're 100% right, you don't get a say in how other people define themselves.
You can absolutely say you prefer to use the new pronouns, just don't make any assumptions. It probably isn't that big a deal. Just admit it was a faux pas and do some self-crit.
Sushi is a rice burrito.