PLEASE HELP!!!!!

ok everyone no judging please?? my best friends boyfriend of three months likes me and i like him. i know im a shitty friend but i cant help it. he wants to have sex with me. i want to have sex with him. but we cant be a couple or tell anyone about it. i really like him but i hate doing this to my best friend. but i realllly like him and we have saved eachother from serious suicide so many times. we just understand eachother. please dont judge me i just want to know what i should do.... thanks x