I think what gets me the most is just how much of an impact Bibi's words have for Amy, because they're from BiBi herself.
This whole series, we're shown that Bibi is frank, blunt, honest, direct, and so on with how she interacts with people. She doesn't think in emotions or in terms of "what creates harmony", but in terms of "what benefits me first and foremost". That self-centered nature of hers means that Amy can fully trust that anything Bibi says is detached from platitudes or niceties or from any place other than cold, hard, selfish logic.
If Bibi thinks it'll help her, then she'll state it. And Amy becoming someone who can make Bibi happy, is good for Bibi - and Amy knows her well enough to know that it's Bibi's truth, and that it's also not impossible to do. Because if it was, Bibi would have already dismissed it as flighty nonsense.
Because normally, someone saying "just change into what you want to be" is next to useless, without the recipient having the conviction to do it. And most people in Amy's position specifically lack that drive and need a lot of prolonged assistance to get there.
It's why Amy dismisses her aunt, and Camilla, and Peter, and Linda, and everyone else before now - they're saying what they think will help, but it's all just talk, and all just what they think will help. None of them have that perspective that allows them to really step into Amy's shoes, and that disconnect means none of them could really help her where it mattered.
Bibi is different, and it's not contrived - from the get go, we're told who she is and how she is, and so this development between her and Amy in this chapter is entirely natural and expected. They're actually perfect for one another in that way, and I mean that in a good sense--sure, Amy's correct in that often, two people struggling with mental health aren't going to make the best partners for one another...but if the specific context of their situations is complementary, then it can work out fine.
And it just so happens that Amy's crippling self-sabotaging lack of self-worth and Bibi's manipulative & pragmatic self-centered nature fit together to see them prop the other up. Bibi needs someone who can care for her, and Amy wants to be that very person--and they can push and pull one another along, supported by mutual love for the other born of actual reciprocated attraction and affection, which is all reinforced by their respective personalities and demeanor.
Makes it funny that Bibi mentions dancing in the hospital room. Because their relationship is like a dance--one leading, one following, but both guiding and supporting the other. It looks crazy from the outside, and is crazy from the inside...but that's what love is, in a way.
Being crazy for someone and being able to smile because of it.
This is easily in the top 5 series I've read in recent years. Just superb character writing and storytelling.