Why do I feel jealous if my best friend spends time with people who unfriended me IRL for my bullies?

I just don't understand. I cannot like those people. And so I get exceptionally jealous when my best friend spends time with them. If people can literally dump me for other people and ignore my existence when their friends are around. Why can't my best friend do the same for me? It hurts me. He said it's because I was busy with work. Yes I was. I was busy. But he didn't need to spend so much time with them. I just don't like it. I can't like it. It bothers me. It really really bothers me.