Finding your voice, done properly, will make you dysphoric. Of course you don't want anyone to hear that! To really know what effect you're creating with your training, you need to record yourself, and listen to the feedback. And that is just, like, the worst! So you want to carve out time just for yourself to do this. And it will take time -- it took me six months of experimentation to nail it, and several months more to strengthen it enough that I could use it 24/7.
But this is more of a relationship issue you have here with your gf. If she's saying it sounds like a guy voice, she's misgendering you. And if she won't give you time and space to do your work, then she is resisting your attempt to transition. The accusations of "cheating" and "dumping" are passive aggressive and insecure... possibly because deep down she fears that your orientation will change and shift as you shed your old persona and step into the real you.