So, I'm dating a straight girl and I'm a trangender female. She has been detaching from me and started to talk to this other dude. What can I do to save my relationship?

I suggest you talk to your girlfriend about this and discuss your relationship. What does the relationship mean to you? What does the relationship mean to her? How serious is the relationship? Those are all question you could ask in the discussion. If she does like that other guy in a romantic way, try to find out (you could just ask her) if she still likes you that way. If not, I'm sorry, but it just wasn't meant to be then. You can't force anybody into a relationship they don't want to be in. But if you both like each other romantically or even love each other, you can make it work together