How do i get my best friend to trust me again?

me and my best friend tel each other everythng , shes friends wit me and my ex bf but but now me and my ex bf are stil close friends and we had sex twice the second time she had found out but i didnt tel her cuz i felt embarriced/shy to tak bout it wit her , and shes mad at me bcuz i didnt tel her dat i did it wit him and she told me i thought we told each other everythng , she said i dnt really tak to her dat much no more and dat i changd and the only thng dat will make me happy is him , and she was mad at him too bcuz of the same reason but he apologized and shes not mad at him no more but i apologizd twice but shes stil mad at me , he told me he tinks dat shes probably more mad at me bcuz she trusted me more but i didnt even kno dat she wantd to kno dat i had sex wit him , and im startn to notice dat i am changn but i dnt lyk my new self if im makn my best friend mad at me and not trust me , i really want to keep takn to both they both mean alot in my life i dnt want to loose none of those two , please sum1 out there kan you help me , give me advice on how to let my best friend trust me again and not let her b mad at me anymore and stil b able to talk to them both and be myself not sum1 whos changn and doesnt want to do anythng bout it........... please help me ...........