How do I overcome my best friend jealousy?

My best friend has recently started becoming close with another one of our friends. He ures me that I am still number one, but he also told me that he tells her his big problems and issues that before he only talked about with me. She likes to rub it in my face that they are friends, and she will constantly poke him and things like that when we are at school (I think she has a crush on him, considering before he came out as a she had liked him for a long time). I cannot seem to overcome my jealously and it is creating problems in our friendship. I will constantly ask him why he needs to joke around with her and if I am his best friend and, understandably, he is very mad. I have tried telling him that I will stop and fix my jealousy problems, which I believe stem from my insecure nature (I am recovering bulimic/self harmer). He will not believe me and is getting to the point of being cynical and mean. I have tried everything to convince him and try to promise him I will be a better friend, but he won't believe me and is insulting me, and I am getting very upset. I really need to get this back on track and any helpful advice would be greatly appreciated.