Nothing about validation makes sense.
Validation is a skill. It took me a while to figure it out, how to validate the valid, so to speak. You are not validating what she is saying, you are validating the feeling. Find something in it that you can empathize with. "It can feel scary to trust. It can be hard to be alone. I have felt that way, too."
You can add other things to help her remember she handled this ok before in the past, and that she has choices. "I am going to the event and I will be back in 2 hours. If you want to come with me, we can stay close. If you chose to stay home, that's ok too. Remember how it worked last time I did this? Everything worked out ok and we had a nice time after I got home."
That kind of thing.
Anyone would think that's just normal and nothing to get upset about. She just kept saying I don't care about what she says and she means nothing to me.
She feels really inadequate and insecure, and is prone to extreme sensitivity and then moods that are hard to control. She struggles to trust, herself the most.
I got so angry defending myself over nothing. So ridiculous. But this is the SCARY part. She does this sometimes and it freaks me out. The more upset I got and the more I defended myself it looked as though she was actually enjoying it.
She sees that you can feel the way she feels, it's a way to get validation. Sometimes, with my BPD loved one, I have found it helps to show more emotion. When my facial expression is neutral and my tone is calm, she interprets it as I am not interested, not listening, not caring.
She started smirking and laughing. Then she started talking in a baby voice and asking "Is (my name) angry? Oh look at you so mad". She said this with a baby voice and laughed after. Then left the house and slammed the door.
It sounds like she may be acting out a punitive parent. When your role changed, hers did too.
I was absolutely raging at that point. All I can think is WOW what is she. Is she possessed by a demon or something?
She is working out unfinished business through her interactions with close others, like you. Some describe this as a type of dissociation, where you are the screen and she is projecting these dynamics onto you and the relationship. We all do this to some extent.
When I tried to explain that everything was going fine until she started a fight with me because I didn't hear her she started saying that she was just trying to tell me how she felt and I got mad at her for no reason.
It is more than just learning to validate, it's about creating a validating environment. She feels inadequate, insecure, unlovable, she struggles to trust others, herself the most. Her moods rocket quickly and it is hard for her to examine anything below the highest emotional watermark. There is definitely more responsibility as her partner to be the emotional leader.