I don’t even know how to start because it’s such a long and complex story, and the title may sound a little dramatic, but I’m desperate and I really hope I may find some guidance from people suffering with bpd. So thank you so much in advance for having the patience to read my story.
It all started in highschool when we first met and shortly after became best friends, although I fell in love with him from the beginning. I may describe those 4 years of school being the most beautiful and adventourous ones, but if I look back, they re full with tears. At that time I didn’t have any idea about bpd and how to act with a person struggling with it, so I did my best with the mind of an adolescent. He was always moody, always impulsive, a small thing could trigger him and he would stay upset with me for days even if I was begging for his forgiveness. But he was also sweet and desperate for love and understanding. He always got my back but he didn’t understand very well what I was going through at times, and he always felt like he was the victim even tho I always tried to protect him and do everything for him.
At 17 I started my first serious relationship with a classmate who was a sweetheart and felt like my soulmate, and he really tried to make me happy, but my mind was somehow stucked on my best friend, even tho I really loved that boy. On the other side, my best friend was devastated with me finding love in another place, confessed his feelings for me and tried everything to “win me back” and interfere, ethic or not. So I hardly broke up with my boyfriend after 2 years and started a relationship with him.
Things were really sweet for the first two years, except for the part where I had to deal with him being impulsive, controlling, jealous and very possesive, but I did understand the roots of this “bad” traits and I always saw beneath the layers - a good heart in need of love, angry on itself. And he was also very aware of the things he did and he always tried to make things up and work on himself. But things got worse, he started to act the same way as he was in highschool and he stopped being affectionate, he would mostly start fights out of nowhere and play the victim.. and the fact that we were doing long distance since the beginning (went to different universities) didn’t help. I am very empathetic and sensitive so those things drained me to the point I ended up having depression, but I didn’t give up.. but after almost 4y of relationship he suddenly left me, without any explanation, and after a month he posted a picture with another girl.
So I’m starting therapy, and after a few sessions my psychologist framed him as bpd and explained to me what this disorder means and how hard it is to live with it and manage it- especially if you don’t know about it. 
Fast forward one year later, it’s May 2021 and his grandmother dies, so I text him to show my condoleances and we start talking again. He wants to meet me, I agree, he tells me that he regretted everything since the beginning and that he doesn’t know why he did this, he tells me how much he always loved me, how he realised what he has lost and how he is determined to make things right. He also shares some of his pain of losing me and the bad things this girl did to him, like cheating, lying and screaming curse words at him. Loving him so much and now knowing about his disorder I’m giving him another shot. He’s all affectionate, he seems to understand my grief and he really works on himself. But 2 months later, he changes his mind again and tells me that he wants us to be just friends until we get to know each other again , he suddenly becomes distant and sarcastic and he accuses me of pushing things between us, not to mention that he still talks to that  girl and thinks it is normal. And now he says he doesn’t understand what he put me through ..and that he felt everything he said when he came back, but now it s not valid anymore.
I’m on cipralex (escitalopram) and xanax since those events happen and I still can barely handle this. It may sound a little dramatic but I was so involved in this rollercoaster for 9 years, I battled depression and anxiety for some time, my feelings for him are so intense and this  conflicted attitude he has even after I gave him a second chance got me down. I can see that his disorder got worse, his inconsistancy, impulsivity, sad feelings, restlessness are way more accentuated ..and I am more determined than ever to stand by his side through all those feelings and help him overcome bpd, I know I can learn methods to make things better and my emotions are so profound, but I’m a mess, and I really don’t know what’s in his mind and if my wishes are achievable.. especially since he refuses to communicate or listen to me, doesn’t think there is anything wrong with him and also refuses the help of a therapist.
It would really help an insight from someone having bpd// loving someone with bpd..

Thank you so much and I really hope all of you will find only happiness and love even in the darkest of days! 

And sorry for the grammatical mistakes, english is not my native language.

TL;DR: F(24y) I don’t know how to deal with the man I love (24,bpd) after he left me for another girl after a 4y relationship, came back, told me he will try to fix things up and now he suddenly wants to be just friends and acts all distant. His inconstancy, impulsivity, victimisation and sad feelings are sky rocketing. I don’t want to leave him especially now that I know he suffers from bpd, but I’m on anti depressants too and I feel like I’m destroying myself. What should I do?