Girlfriend hard to understand? please help me - wanting to start a life together but don't want problems?

My gf is pregnant she gets out of control sometimes even throwing things at me. Yes I do argue with her and sometimes say mean things but correct in the current situation and she's not always wrong, but what she says usually doesn't help the case any or even make a dent in what we are arguing about, an example would be thinking shes ugly, I will sit and tell her fiftythousand times over and over in different sentences,as many ways as possible but she doesn't budge even when me plus tons of her frnds tell her shes pretty. almost anytime we argue it seems she gets off the tracks of the argument, finds any itty bitty anything and sticks to it, even when i will sit there and tell her how and why it's not true in the most childish easiest to understand way possible, yet still no budging. Later she usually tells me shes sorry, I know that she truly is.. but how do i deal with it? I love her and need her in my life.. the stress on me is so hard to deal with how can i make her think logically