Wow, that was fucking ridiculous.
She's told right from the start that Cecelia can't be trusted, then tells us after trusting her that she just didn't imagine that it could have been a misunderstanding. In fact, it's not even a misunderstanding, Amanda herself says she was shown documents out of content, she understands now that it was a complete lie and a manipulation of the facts. Yet, all Amanda is will to admit to is being selfish, because she wanted the MC to tell her the truth rather than asking.
Like, even teenagers aren't this dense and willful that if you tell them a fire is hot that they'll just go and stick their hand in. This is beyond childish naivety. And what's worse is that even if you choose to tell her she is being selfish the MC's response is 'yes, but I don't care." The story is trying to railroad you into a preferred route where she 'did nothing wrong'. Like even if I want to continue the relationship and rebuild there should still be some rebuilding of trust and consequences, not 'walk all over me and I'll still love you'. It doesn't make Amanda seem immature, it makes the MC seem immature.