I'm so sorry you're going through this, Avatar

It's difficult and I totally understand.
Maybe try to explain it to your cat in its cat language. Hopefully she'll understand. I would be devastated to part ways with my cat. Communication will help
a lot between the 3 of you. Just talk it through and see how it goes from there and hopefully everything gets resolved.
Maybe ask some questions to your cat like, "why don't you like my boyfriend?", "do you have allergies to certain humans?", "is it your cat senses that something isn't right?". Then ask your boyfriend some questions like, "are you a cat or dawg person?", "are you allergic to cats?", "have you ever had a cat?".
Also another thing I should point out is that is your boyfriend's nightmares seem like a really big problem that can spiral into something worse. Just be there for him at times like this - you don't want it to worsen.
I hope things work out between the 3 of you. Keep us updated.
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"People say nothing is impossible, but I do nothing every day." - Winnie the Pooh