Girlfriends loves me but also likes my best friend and he likes her back?

Me and my girlfriend's relationship is well getting rocky...we almost broke up but we pulled through and she seemed happy. then i realized she likes my best friend and she told me, and he likes her too. I have been getting a little boring but i love her and i always will. Im not really an assertive guy and twice something happened but i didnt do much.. (partially becuase it was my friends fualt) anyways she and my best friend started talking and they find out they like each other. hes like me but to her hes more exciting and more assertive and "balls-ier" than me. She came to me and talked about it and she really likes him even though were less than a week from 6 months together. At first i felt betrayed and i still do, but i want both of them to be happy but im human i want her and they both dont want me to get hurt.He said no i wont go out with her(but i have talked to him...this is what he said to my girlfriend) what should i do i dont know if its worth going out with her when she like my best friend or feeling the pain everytime i see them...please i just a word of advice