How to write first-person POV and stay true to that character’s personality without letting your own personality get in the way?
You most likely can’t, but don’t worry about it.
Think about it. Readers know who wrote a book without being given the author’s name because a writer’s personality comes through in their writing. If it didn’t, they’d be a pretty poor writer.
The writer’s personality is what gives their writing its style.
So when you write first person, you’re actually writing with two voices. One is the voice of your character and if you know your character that voice will come through loud and clear.
The second voice is yours and your readers want that voice just as much as they want the voice of your character.