From all I've read, it's just a matter of practice. All the tutorials I've seen say to use falsetto a lot to train the muscles at higher pitches. It's learning the feel of the vibration in "the head" rather than from the chest and the feeling of the adam's apple being squeezed up into the back of the throat. Then it's a matter of lowering the pitch down to a normal speaking level while retaining that "squeeze" and "head voice" which should result in a feminine voice. Most people have practiced this for months before becoming consistent and passable but there is no shortage of great girl voices coming from someone with a deep natural voice.
It's possible to train the vocal range to very impressive lengths. I've heard of sopranos being able to cover 6 octaves, which is ridiculous, but you should be able to train yourself up without too much trouble.
My natural voice is fairly deep but it doesn't sound like it's as deep as yours; I can do full-on death metal growling, though, so we aren't talking about an average level. Despite this, I am coming along. I have not been practicing for long but I have spent years practicing singing in general so I have good voice control.
What I've noticed, however, is that the average cis girl's voice isn't actually that high. I've been paying special attention to how cis women speak and it's more about being unbroken and speaking melodically than being high pitched.
