But she has not turned her back to him? Play Chapter 14 again. She's "in between". We feel that makes sense and realistic for a girl her age.
That's pretty much the problem.
While you have men that love to say they like younger women, what they don't want is to deal with their immature bullshit.
Now the issue is you've put a girl in the position to be hated by the people that prefer other characters. On top of that you've also highlighted the biggest issue of dating someone so young, they can't handle, well, anything.
People don't like dealing with other peoples shit. In a relationship, if this was to happen, some people would count their losses and move on.
The one major flaw of using young women in roles like this is they aren't ready for that kind of commitment. All this has shown is Amanda isn't ready to handle a relationship or deal with tough situations on her own.
To put it plainly, you've taken a character that was portrayed as happy, lively and loving and turned her into the kind of person some of us just don't have the patience to deal with.
As I said, it doesn't matter what the aunt says, what evidence she has hell, it wouldn't even matter if the MC was guilty of it. Amanda has kinda been pushed to the back of the girl queue by a few people here for characters they prefer. For some, she was never going to win against Heidi or Kathy and now with her acting like a child she's even further behind in the running with no way of her catching back up.
Which leaves the issue for some of us that can see where this is going, as it happens with all romance novels with "oh no, drama, oh yay we made it through" situations where you have the MC and Amanda live happily ever after while you have readers that are praying he ends up with one of the others.
While i'm not a writer so can't tell anyone how to do it properly one suggestion I would give out is, if you are going to make the main love interested have such a flawed personality that grates on some people .... make sure that MC doesn't have better options he could be going for because you can bet your arse your readers will be hoping he picks one of the others.