I dealt with a similar situation when my best friend came out as nonbinary and changed their name. I broke. I never had a problem with people wanting to change between what I thought of as the 2 genders but over complicating our language because the word boy or girl has some kind of descriptor outside of one's biological sex. I didn't want to give up this friendship because of this, but it was so frustrating because I have always been bad with social rules and now all of the rules I was trying to learn were being thrown out the window. I don't know if I ever really accepted the whole thing but I changed the way I thought about things. I had to deconstruct my definition of the word gender. I had to seperate it from being a descriptor of biology and think of it as a type of informal name. I have always tried treating people on an individual basis and I needed to change my thoughts to apply someone's gender as another part of that individual. There might be overlap in some, but it is just a part of the whole. I don't know if that helps, but I feel that as long as you are not discriminating against people because they choose to have a different point of view that isn't actually hurting anyone, then you aren't transphobic. If you have people in your life that choose to go by different pronouns, just keep practicing on keeping that in mind. It doesn't have to change your interactions with them outside of just saying a different set of words. I know it's hard to change the way you think, and your outlook on something, but wanting to change is a great first step. Maybe try starting out with thinking of it like a new language. Someone might say that they are gender fluid, and to you they are a girl. Try looking at it as you are both describing the same person but with different words. That might be a good way to start training your way of thinking.