I'm going to side with my dad after he berated my girlfriend
Why would you side with your dad in this?
Because my dad is one of the most important figures in my life and I'm afraid of losing him.
Your dad may have had his own reasons for berating your gf, but it doesn't make it okay. Not being on her side to defend her in this case is not correct. Don't let your dad get away with inconsiderate behaviour.
But I need to make sure my dad will always be on my side.
I think it's important to consider your girlfriend's feelings in this, not your fathers.