Please don't assign an intention to my words that isn't there.
I did suggest the OP get therapy. I did so because the transitioning relative is over 18 and therefore it's their decision, not the OPs, and the OP would benefit from having help working through their own emotions surrounding it. Whether the OP agrees or not, whether we all agree about the support systems offered to trans individuals, or choice of pronouns, or what the outcome of a transition really is -- it's not the OP's choice to make.
There is a sort of bereavement for the family when someone transitions; the person who existed prior to a transition is in a way, no longer there. There's grief and loss for the family, and a definite pain to letting go of the narrative family members build up for that person in their head.
That is what the OP can discuss with a therapist, not "how can I accept this decision?" because acceptance may never come -- that is one of the things the OP does have a measure of control over. It's not "what is wrong with me because I can't accept this", it's "How can I handle my own personal emotions around the transition and my relative?" It becomes particularly important if you don't agree with the decision to transition and at the same time you'd like to preserve a relationship.
OP, I get it. I know it feels like right now your niece is staking future happiness and making an enormous life decision based on something that has varying levels of success. It probably will, at some point, feel like they are SO OBSESSED with the idea of transitioning that it's become a fixation and you worry about what is going to happen when it's not the panacea you think they've built it to be in their minds.
I have been there. I've thought probably every awful thing you have and likely a few more. The only advice I can give to you is to find that impartial ear to help you process all of this, because you need to grieve and discover what it is you do have control over and how to bring that in line with how you feel -- and find your way forward from there.