I’ve learned quite a few things from reading book reviews. One is that readers don’t like selfish or whiny protagonists, and they really don’t like protagonists who are selfish AND whiny.
Actually giving protagonists those characteristics as flaws is very interesting, as long as it’s deliberate. No character can be perfect. The thing is to have other characters notice these flaws and call them out on it.
Unfortunately, characters can come across as selfish and whiny whether you intend it or not.
Isabel has quite a good reason to be angry and resentful – at least, I hope she does. I don’t know how to let that come across without overdoing it. She also finds it quite difficult to forgive.
As for the selfishness…she ends up in a position she’s completely unprepared for because she was never supposed to have it. She doesn’t say “I don’t want this, let someone else have it”. She accepts her situation and resolves to do the best she can. She’s determined to be strong.
Unfortunately, what she thinks being strong is hiding her emotions and putting up a veneer that says you don’t want to mess with me. She’s afraid to be vulnerable, and she doesn’t want to be afraid.
Hopefully she won’t accidentally come across as a horrible brat. But you can’t please everyone.