boyfriend vs. best friend (guy)

At the beginning I told my bf that my best friend used to like but nothing happened cause I didn't see him like that. I wanted to be honest with him, now 8 months later he's still getting jealous of him. I've hanged with my best friend behind his back. Whenever I would be honest with him we would always get into an argument "you care more about him" so I was done with the arguing. So recently, I realize that I shouldn't be lying to him because I wouldn't want him doing the same to me. So I decided to cut my best friend off because I don't want my boyfriend to feel like I don't care about him.I thought it was going to be ok but then I get a msg from my best friend: 

How do you think I felt when I read this! So I realized that I made a mistake. I sent this to my boyfriend and told him how I felt but he didn't understand. My bf is the sweetest most caring guy I know, so don't think he's a bad guy. He just doesn't like my best friend at all. He trusts me but not my best friend.

I don't know what to do, I'm stuck and I need help.