What do I do? *Guy Probs../:?

so, i have this boyfriend.. i love him to death. but, he has this BEST FRIEND, Cheyenne.. shes popular, and so is he. shes pretty, to. me and cheyenne dont get along. i dont want to have to go through his fb, but i do to check on him. theyre always like "hey bff!" and he acts flirty with her. she likes him. on his fb, she wrote "would I date you? yeahh..." also she wrote "do i love you? YEAAH, Best friennd ! :)" and he doesnt say anything to her, like, "woh. i have a gf". and he says he doesnt like her, shes hot n stuff, so idk y he doesnt brake up with me for her, he says he loves me. and he once said he liked her, but then texted quickly "nevermind!!" idk what to do.. when i see their conversations i wanna cry... i mean, i USED to have this best friend brandon, we always hung out cuz i had more classes with him then my bf. and he always said he was jealous but i never knew how bad it hurt, am i feeling the same pain he did? i cant cope with it anymore. i really need help, but i really love him.. id kill myself for him. and he says he loves me, and he cares.. it really seems like he does. but i think he loves cheyenne too.. but he has A LOT of best girl friends.. i dont have a lot of best guy friends, at all. like, i threatened to kill another girl that was his friend.. and im a 6th grader.. you may think thats too young for love.. and id just say i agree, but what we have.. is just, great. it feels good to know someone loves you. please help. and leave an answer, not just like "idk thats ur desicion" cuz i cant make one /: help, please.