Hey mrperfect85
First, Welcome to the boards
most of the time she throws them back. So she basically said it’s because she feels insecure
This is a big issue when interacting with pwBPD, they're emotionally sensitive people (especially to perceived negativity) and they often tint their interactions based on emotion. Unfortunately those emotions are often about feeling bad about themselves, so it can be hard to navigate.
But if she has a reason like feeling insecure or feels ugly then I understand, and I would still compliment her
Be mindful that "agreeing" with her about the insecurity can make her think you also think that there's something wrong with her. Its a twisted perception, but it is very real to them.
she’s very mean to me playing around all the time and being mean regularly
I'm interested in this part. We fell in love with them for a reason (I was in an on/off relationship with a woman with BPD) and I understand that you're letting her know the good that you see in her with your compliments.
My gf also knew about her bad behavior and we would talk about it, and she could be very hurtful in the things she said and did.
I hope you don't mind me asking, but in what ways is she being mean to you? Has this been going on for long or is a recent development?