As as an FTM femboy, I'd feel disrespected and unseen if someone called me she/her behind my back like you are essentially doing to Hazel. Seeing as Hazel is human, not a non-human alien or fantasy world character, surely you can understand how this is flat out inappropriate when applied to actual humans outside of fiction. You don't get to decide someone's identity and pronouns for them. There's no "personal" choice for you, especially when it comes to something that gives people discomfort or worse, anxiety and dysphoria. Just be honest and say you want to pretend Hazel is a girl.
Edit to add that a man's enjoyment of feminine things and presentation (same situation for anyone else really) is not an invitation to call them by pronouns they don't use.