Cuckqueaning's something that should be enjoyable for everyone, so if you're there all torn up inside while your dick's in the vixen then that needs fixing. Everyone assumes that cuckqueaning's something a man will enjoy no matter what it's like, but that simply isn't true and you've as much a right to not have to participate in something that squicks you out as the girls do.
>It's my concern for her, personally, I can't see the comments as anything other than mental self-harm.
I'm a touch confused. You think that maybe your GF's lying about liking it, or that she likes it but that it'll damage her, or that she likes it but shouldn't? I mean, your question was
>How do I stop feeling so overprotective?
which is about your idea of her wellbeing rather than her wellbeing, if that makes sense.
On the face of it, it sounds like your GF and the vixen felt their way into a humiliation-driven dynamic that both of them seem to be enjoying, but that you're not. Assuming this enjoyment is genuine on both their parts and they wouldn't also enjoy a less humiliating, compersive dynamic just as much, you can learn to enjoy that dynamic, change it, or not participate any more.
Each of those three options has a lot of ways you can go about it, all of which will depend on a lot of different things, so I'm just going to give some random ideas.
First and most obvious is
>Talk to your GF about it
I don't know how things are between you, but "it's weird, I know it's roleplay and you say you enjoy it, but I can't stop feeling uncomfortable and like I should protect you every time <vixen> says something mean, and it's throwing me off during sex"
seems like an okay way to start the conversation. Dig into the details and figure out what specifically is squicking you out and why. Listen to what she has to say (assuming she's a good communicator who knows how to honestly talk about sex; not all women are) and see if together you can figure something out.
What it comes down to is
>Why do they like it?
>Why don't you like it?
>Can you do something else instead that everyone likes?
Some other ideas on how you might shift things around:
Probably not going to work since you said that you
>can't see the comments as anything other than mental self-harm
but given that the vixen likes to tongue-lash your GF and your GF likes it, you could join in on the verbal bullying to help get your GF off. Sometimes participating in something and seeing the other person enjoying it helps to lessen anxiety because then the nature of the game is obvious. Maybe your GF would like a little bit of verbal humiliation from you too when the vixen isn't around, as a treat?
>Act as counterweight
LOCAL VIXEN NOT SO TOUGH AFTER BEING DOMINATED
This is also known as the "that's my
girl you're talking to" solution. The vixen likes lording it over your GF? Remind the vixen who's in charge. (You. It's you; you're in charge, unless this is some kind of vixen-femdom thing in which case all the suggestions go out the window.) Vixen says she's the better woman? Grab her by the throat - or whatever suits everyone's boundaries - and have her prove it. Maybe everyone will like you taking charge like this so much that they won't miss the insults. This turns the vixen's verbal humiliation into a kind of bratting play, and your responding to it that way might satisfy your need to protect? But it also might turn into an escalating bedroom struggle-game, which I imagine would be exhausting if you're not into it.
>Figure out an alternative
The verbal humiliation squicks you. Is there some other and less verbal way the vixen can get her sadism fix and your GF her masochism fix?