Regret: Date before Mate.

About a year ago my best friend was and is in love with this guy that i used to date, it was a huge love triangle for about 2 years. Everytime he would choose me over her and i didn't care bout her feelings. After about a year of them not talking at all they went out again after me and him had broken up for like the 4th time or something. I was jelous and i couldn't stand it. I wanted to break them up but then i felt bad because my bestie loves/loved him so much and she had been through alot. One day after school i went to my bestiez house and her boyfriend (my ex) came over as well, the majority of the time me and him were play fighting on my besties bed. Later on my bestie n her boyfriend were argueing next minute he says "ur dumped". My bestie layed shattered on the floor and he was lying on her bed. I was stuck in between but i tried comforting both of them at once. While my bestie was crying on the floor i lyed on top of my ex and kissed him. It wasn't untill my bestie and i had a huge fight that she told me that she knew everything that had happened. i regret it so much cause i love her. and he treated me like shit and i can't believe i chose him over her. I've changed now. But the thing i hate the most is that they're going out now and she hurts her mentally and physically and she wont do anything about it. It doesn't matter what i say. She's so in love with him to listen even though she doesn't deserve everything he does to her. I have a boyfriend now.. happerly in love but i can never forgive myself for what i did in the past.