Sharing Our Innermost Thoughts

Is it wrong to become a bestfriend of someone who is already someone else’s bestfriend?..Actually,this boy and girl who were besties brokeup because he became more close to me as a friend.All 3 of us are classmates. When they had a fight and were not talking to each other, he sent me a request in Fb n we started chatting. We were having study leave then when we chatted daily n i started feeling more close to him as a friend, more like a best friend. He was a fun loving and cheerful person. But aftr the exams when classes began, he started talking to his other friend and he had to be with her more. He used to talk to me casually, but she had started feeling possessive for him. He had this habit of making her feel jealous by telling her abt me. So i tried to stay away from him,but I thn started missing him.Once when they had a fight on my name, i talked to him and solved the prblm telling him that i wont come in between them.But that day when i saw them sitting together in the bus, i felt like crying n had to control tears by pretending to be listening music. He understood this so he continued talking to me as usual. As we continued to talk and due to his habit, she had to beak up with him. But those days i had asked him several tyms to solve the issue.He was stubborn that he wont ever go to her and told me that if i had a prblm with him, he was ready to leave. Since those days i have this guilty feeling coz one of the reason for their breakup was me. Aftr 4 months, he proposed me n now we r in a relationship. During the initial days of the relationship he always compared me with his ex friend in everything including intimate things.He always made me jealous purposefully by talking about her. I used to be very much sad those days. Aftr 2months he started talking to his exfriend,which i had no problm with. But due to his habit of making me jealous,i started to have problem with her. At the end i had to tell him that if he talked to her ever again we wud have to break up. He stopped talking to her and visiting her home. But still this guilt feeling is still persisting. He confessed to me that he was not in love wen he proposed me but had felt jealous and insecure when i talked to another guy who had one way luv towards me and that he still missed the other friend those days. But i luvd him so much evn b4 he had proposed me n was not able to breakup wid him.I went thru all his comparisons and now he tells me that he luvs me but i cant feel evn a little bit of the luv he had shown to his other friend.We always have fights and he always does this casual body shaming due to which i have startd feeling insecure abt my body. I cant leave him coz i luv him but evn now i cant feel that he is mine. Whenevr i think of him as mine, his other friend’s face comes to my mind.I had discussed all these issues with him several tyms but he doesnt seem to understnd anything. I feel so much guilt which becomes one of the reason y i dont wanna leave him.I dont knw wat to do now.I think this other friend still wants to talk to him but she is reluctant thinking abt me.He tells me that he has strtd hating her for few reasons…idk…some1 pls help me in finding a solution to this.