I Think Failed Relationships Change People

I don't know what to do ...I feel sad and I cant even know what is the reason exactly.
10 months ago I have met someone in one of the social media and I loved him so much.
otherwise he treated me bad and he used to yell at me. we saw each other three times. He is away now and we are not together anymore but his best friend is in love with me and when he knew that.They have lost their friendship. I didn't do anything wrong and I started to talk to his friend when we broke up . He left me in hurt and he broke my heart. He cheated on me and never was honest with me which makes me confused and sad because I really have loved him. I don't know why do I feel guilty because they have lost their friendship because of me. I mean it doesn't make any sense. he is no longer in love with me .. so why did he get mad when he knew that his friend likes me ?
I am confused and I feel gloomy. Cant focus on my study , thinking a loooot and I don't know if I miss him because he just destroyed everything between us and he is blaming me for everything!
sorry for my bad English I used to be good in writing however its been along time since I practiced.