It took me a year to be able to hold a normal conversation with my voice. The thing is, it is not just changing your pitch, but also your speech patterns and inflections. You are already used to speaking a certain way with a certain vocal pitch, and if you continue to speak that particular way, then your pitch will fall into its natural range.
I have a few tips you can try:
The first is an exercise that I did all day everyday (in the car, in the shower, in my room, etc., etc.). I would repeat the line "Mia was driving" and have my voice come out of my mouth and nose, rather than my chest (which is what causes the low resonance in a man's voice). There are plenty of videos explaining this technique if you google voice training. I'm not sure what techniques you are using but I would check these out if you haven't already.
The second is having realistic expectations in terms of how high your voice will actually get. If you are already past puberty there is only so high your voice will be able to go, but this does not mean you can't sound female! As I said before, it is all in the inflections and patterns of speech. Watch movies and TV shows and observe how the actresses in them use their voices and imitate their inflections and behaviors. Also, find a "voice model" you can emulate. I chose actresses Laura Prepon (she plays Alex in Orange is the New Black) and Zooey Deschanel, because they have fairly deep voices and I found that I could pretty easily match their pitch and maintain it, then I observed how they spoke and based my inflections off of that. I knew I was successful when a customer at work told me I sounded like Laura Prepon lol
Hang in there, it is one of the hardest things to change, but like they say, practice makes perfect! And I really mean it when I say that observing other women in real life and media is so so so helpful! None of us have grown up in a vacuum, we all pick up our behaviors through observing others. It is the same for trans people, the difference is that what cis women have learned at age 5 some of us are only just starting to learn past puberty.