I know understand what is going on.
Wish-Nikki synched wonderfully with Bear until Ian told him the truth about her. Ian undermined his fantasy. When he confronted Wish-Nikki, that was the beginning of the end. The Wish-Nikki starts destablising, as the real-Nikki starts breaking through. Even though Bear says, "it doesn't matter" (ie what she says about her father), his actions, mindset has altered as he can't just unlearn that, no matter how desperately he wants it to work.
Evidence 1: Real Nikki breaks through on the initial night BECAUSE Bear could not accept her behaviour and was not ready for it.
Evidence 2: When wish-Nikki approaches him the next night, explains herself, Bear accepts she is real, and he confesses he likes her. Then you have a positive feedback loop and the real-Nikki is suppressed while the wish-Nikki is dominant, because Bear of himself is stable.
(example of positive feed-back loop:
The montage may be a fraction of the screen-time of the movie. But it at least shows there is no rupture of truth (ie about Nikki's father) during that time. Wish-Nikki is stable -- a little suffocating eventually as time continues with Bear, but Bear is 100% comfortable with wish-Nikki during this time. The horror aspect of the movie is the next sequence over when Ian destabalizes Bear to doubt Wish-Nikki.
Most people simply miss this sequence within the movie that what Bear believed he had during this time was real and he created a new identity around her. Wish-Nikki knows him and her own stability is contingent on his stability. Since he is stable, they are both happy.
Evidence 3: Bear gets bad information from Ian that Nikki lied about her father. This is the seed that destroyed Bears ability to maintain the fantasy -- even if he agrees "it doesn't matter", Wish-Nikki is attune to Bear, his subsconcious, his feelings, and know "something is wrong".
This destablizes her and the real-Nikki starts breaking through, while wish-Nikki starts behaving more bizarre and strange in ways that were not part of the original montage. This reflects the cognitive dissonance Bear feels. In this sense, wish-Nikki is a mirror of Bear's internal state. We are seeing the collpase of Bear's fantasy in real time as the feedback loop changes to a negative feedback loop.
Evidence 4: The more dramatically destablized Bear gets, the more real-Nikki breaks through. Peak destablization occurs in Bear when he had that hotline and heard Nikki screaming. The next sequence is the real-Nikki begging him to kill him. It reflects that Bear of himself can not internally maintain the fantasy it is now a negative feed-back loop (ie real-Nikki break throughs and/or wish-Nikki behaving more erratic and dangerous as Bear's instablity is now mirroring on her)
My ex-girlfriend also appeared to be very sensitive to my state. When I started getting more information that destablizied the relationship (ie her past, whether her divorce, while theologically incompatable, was something I could continue to live with as I learned more of the details/parciulars about it), I could not unlearn anything. She detected something changed with me. Even if I wanted to continue it, it would not be the same since the parameters of it would change with the new knowledge.
Ian therefore killed the whole thing in the movie, as it shows that it destablized him to the core, no matter how much he wanted to be with wish-Nikki.