Working at a call center for 10 hours a day and talking all of that time, my female voice developed really well. I am confident that the most passable thing about me is my voice, I never need to announce my name I am always gendered properly. Why am I here to tell you all of that? Because I wasn't always this way and struggled with the voice, but I am living proof that if you practice it non stop and don't let up that it will happen. A few tips if I may.
Stay out of your male tone if possible, try and let it become defunct through atrophy (not using it) Some trans like to toggle back and forth between boy and girl voice but its my experience that it is more successful staying in female.
Don't stop or take too long a break. Having spent half a year full time out at work and speaking with my female voice for thousands of calls, I notice the more I am doing it daily, the better I am. I had some week long vacations in this timeframe and I noticed when I stopped using the female voice it would slowly go away and I'd have to work it back up. A female voice is not natural for us unfortunately, at least until you make it natural.
Always be aware of your voice, by this I mean when you're alone and talking to yourself don't let yourself rest on your male voice, make it a habit.
Don't be afraid to experiment and find your voice, even with months of practice I am still picking up different inflections and words that I like. Don't be afraid to mix things up with your voice.
You don't have to have just one voice, you can have multiple female voices. I have my more relaxed and lower pitched leisure voice and then I have my higher pitched professional voice, and then I have my super high valley girl pitch which I use for fun. Just like a cis female you have more than just one range, so don't be afraid if you discover more than one.
Last thing, make yourself become long winded in some way shape or form. For me I want to keep my female voice worked out everyday, so when I don't have work and I tend to want to just keep to myself I have to find some ways to keep my mouth going. So I started a video blog where on my days off I will spend at least a half hour talking.
So there it is, us girl's become too hard on ourselves and think we cant sound feminine when in reality we can, it just takes unrelenting determination to where you make yourself never give up. Hope this helps.