I’m sorry for the long post, but i really wanna understand.

Couple years ago i started dating my best friend, she was this amazing charming girl that i was so in love with.

When we got in the relationship everything changed,I was controlled, obsessive jealousy, i wasn’t allowed to talk to other girls, even colleagues, physical abuse, scary rage, threats of self harm, swallowing pills, and sending me videos of her cutting herself when we fight, i used to walk on eggshells, scared of her reactions. 
She comes up with a scenario in her head and believes its true, she even accused me of cheating because she saw a picture for me and my female friend on social media before we even started dating. 

Now i couldn’t tolerate all that, so i decided to leave (i ended it in a text because i was scared from her tbh), she didn’t take it well, she kept trying to have me back for 5 months, using her friends, getting new numbers, new social media accounts, and i kept in touch with her then i had to cut it off fully.

When i cut her off she got in a relationship with a guy a week later (December), maybe less than a week even, IT WAS ALL OVER SOCIAL MEDIA, seemed so happy with him.
i was shattered, i didn’t leave because i stopped loving her, i just couldn’t tolerate the abuse.

On March she reached out to (we are in the same college) to say sorry and all
She told me she got diagnosed with a PD (her therapist didn’t specify what it is) and i am not diagnosing here but i saw all the 9 traits of BPD.
Also diagnosed her with Bipolar, PTSD, ADHD, eating disorder, depression, anxiety. 

She told me that she is in love with me, that she is not in love with the other guy, that she misses me, she started crying blaming me that i ruined our “beautiful relationship” and that she would leave him if we get back together.
She told me that she uploaded a picture of herself wearing a dress and added all my friends so i could see the picture.  
She even sexually teased me (and i fell for it which absolutely wrong and i regret that a lot).

She kept texting me behind the new guy’s back 
And flirts with me and sending me pictures of herself and sees me in college ( and i am guilty myself for allowing all of this, i wasn’t thinking clearly), then suddenly tells me how much she adores the new guy and loves him and that she wants to marry him, then tells me how much she loves me and asks to see me. 

All of a sudden she disappeared, she stopped texting, or asking to see me or anything, and i stopped too. yet i see her view my IG stories sometimes and the disappears also.
(Its been almost a month since the last time we talked or sat together.) 

I still see her in College and we say hi and thats it. 

I am living my life normally and everything is okay, i just want to understand why all of this? 
Why did she behave like this?