She's in full denial, I feel it makes sense, this is basically a fight, flight and freeze situation, think about it, she's literally just a dumb teenager dealing with an absurd situation, without no fucking clue of why and how everything is happening
I assume the dude went through similar situation before he decided to fight against it, though probably not as bad
It's possible she's also unsure of her own sanity, after all put yourself in her position, she's basically in the real world and then some absurd seemingly impossible thing happens, nobody else sees it nor would believe her, what is more likely, that the monster is real or that both of them are just going crazy?
I imagine next chapter or so will explain why she gave up, I don't think she's being affected by a parasite but it did just change hosts, it could have muted or something