Should I forgive my bestfriend?.. Please help.?

Okay, so I've been datin this guy off and on for about 2 years now, and me and my bestfriend have our lil girl talks where we talk about how much we really like these guys and stuff, and me and her are really close, we play sports together, we hangout alot, just like bestfriends should. Well we just recently got out for school for summer and this rumor gets around that my bestfriend, and the guy I'm totally in love with have had sex.. of course i didnt wanna beleive it, but the guy( the one i've been in love with forever) kept tellin me it was true.. so finally i asked my bestfriend about it. of course she said it was a lie. and that he only startted the rumor to make me mad. so about a week goes by and i stuck with the story that he lied and she really didnt have sex with him.. then i get this text from my bestfriends ex boyfriend sayin that my bestfriend really did have sex with the guy i like and that she just didnt know how to tell me. so here i go again askin her if its true, and she says we gotta talk. so she came to my house and we had this long conversation about it all. she said she really did do him and she was truely sorry. she said she just got caught up in the moment and didnt know what she was gettin her self into.. should i be mad at her still? or should i be mad at the guy i like? or what i need answers please! plus all my close friends that are guys are tellin me how much of an idiot i am for not being mad at my bestfriend.. I mean i just dont know what to do. please help! :((