my(15m) girlfriend(14f) has bpd and i need advice from others who live with it.

(online relationship we've known each other for 5-6 years) 

so in the past i was a disgusting person. i would talk and act sexually to everything and anything. and i know it was all fake (doesn't excuse what i did i know that) and my girlfriend still struggles with believing the truth. that i love her AND ONLY HER!!! and that i don't give a shit about the people she thinks i like, i don't like them, they disgust me, they anger me. i hate myself for what i put her through. she hates herself because she thinks she will never be good enough for me. i've loved her for a very long time, i regret what i did and how i acted. 

today we were having a good day (i thought) we were laughing, smiling, joking and just having a good time. then she went on tiktok to ruin her mood and look at pretty girl. i kept telling her to not go on tiktok, and to not ruin her mood. she told me she can't control it, i don't really understand so please help me with that. this is something that affects our relationship (9months) and i've been trying to fix for about maybe half of the relationship or more. how do i help her believe the truth and help herself? why is she doing this to herself, she says she cant control it and i want to understand, listen and be considerate. i love her with all my heart and i want her to know it. our relationship is very complicated. we can't communicate that well because i cry because i feel guilty and that makes her feel guilty for making me cry, she blames herself for everything and i just want to make her happy. 

why does she do this to herself, and how can i help?