This is so difficult. I think @LadybirdsProcessing\s core point is really good, but I think you are really really going to have to focus on Not Going There too.
I am ND and I feel that you may also be too? Or perhaps just have similar traits in that you find the thought of not telling the truth at all times really hard to bear. It feels inauthentic, like 'not being yourself', and almost physically impossible - but you can teach yourself to do this and honestly, it starts to feel ok. It's not that you are lying - your daughter knows what you think - it's that you are choosing where to tell it. (TBH this has been good for me as a general skill haha)
If I were you I would be self-soothing/-controlling with the point that @VikingLady made:
"Id probably be NB if I was 14 now, just to take myself out of the whole mess."
Try and view it from this lofty historicist perspective. It doesn't change the sadness but it makes it less personal. This is not really young people's fault (with some reservations), and not DD's GF's fault. She's looking for a lifeboat - I think very few women would have their breasts removed unless things were very, very bad for them, even though you disagree intensely in terms of your diagnosis of what, exactly, the bad thing is.
To build on what @LadybirdsProcessing said - in every interaction I'd force myself to say something upbeat, positive, trusting or open. "LOVE your trousers GF" :That's a really good point GF." "What shall I watch next on TV, I've come to the end of my drama". "God, I'm so happy the weather has turned, I was honestly getting a bit depressed."
I have noticed that when there is some positive communication, the conflict necessarily becomes less dominant. One can feel it taking up less space/time, pushed out and down by the positive relational things. It's one of those annoying slightly CBT 'wellbeing' things that turns out to be less bullshit than the rest true. One's own mood is better and so is theirs and, crucially, it's another way to show that while you won't change - you haven't changed.
My other self-soothing mechanisms would be to remind myself at all times:
*DD hasn't been forced by the Kool Aid to change herself - she is a lesbian and she is going out with a woman, whatever she herself believes.
*By the same token, you know that the GF is a woman so forcefully think of her as a woman at all times. Don't engage with any thoughts of your own or conversations with the girlfriend that don't fit with that truth. Equally, don't allow her to bring up anything to do with Sex - you wouldn't talk about periods or whatever with your DD's girlfriend, don't talk about her moustache.
*If she were going out with someone non-trans who had odd views, you would also be having to bite your lip. You would. It would be horribly uncomfortable but you would force yourself to do so to avoid a crisis point. So dont' allow your justified sense of being socially/politically under fire as a SR to make this any more significant than if your DD's partner was, say, pro-Brexit or homophobic or conservative-religious.
*If you make this a Me or Them issue, you will have given Them what they want and you refuse, absolutely refuse to allow Them that power.
*The only thing that is permanent is Change.