I like that the author is portraying mental illness in a realistic way. Trying to change when you're stuck in a toxic mindset (especially one that has been enforced by the people around you for all of your life) can be difficult, frustrating, and painfully slow. It's impossible for her to just think "okay, enough of this, I'm going to start standing up for myself now". She spent her whole life being told by her father that she is worthless - it will take a lot of time for her to understand that she is worthy of love, even if she's not "the perfect lady". Right now, Maxi is trying to change, but even her reason for trying to change is unhealthy - she thinks that if she does something to become "a good lady", then maybe when (not if) Riftan stops loving her, he won't abandon her. It's really sad, but it makes a lot of sense, given her past, that she wouldn't even dare to dream that Riftan will love her all her life. I hope that Riftan gives her the unconditional love she deserves, so that she eventually realizes that she is as worthy of love as anyone else.
tl/dr: It makes sense that you find Maxi frustrating; healing from trauma is a slow and frustrating process. But when she does start to heal, it will be even more heart-warming because that slow process makes it much more realistic.