My girl cheated on me,but she refuses to tell me she did,even though all the proof is there?

this girl i talk to everyday, went outta town for 2 months.and on her fb,i see she hacked a guy`s page something,and it was like i love you.and she put a love sign on it,but ina special way,i know she writes stuff.so i ask the guy,is anything going on,cause she told me shes not with him.the dude said,no man,we are just good friends,im with her sister,but my gut was tellin me otherwise.im very pissed though,because,all the proof is there tht she was messing with himon fb.and they got into it,and she sent metheir message,now in the message,it showed one good detail tht i saw.they were talking,and the guy said,i am happywith my girl,and ijust wanted to be cool,and he cant do this anymore.so i ask her whts all tht about,everytimei ask her,certail question about something,she gets mad.she says she dont like liers,cheaters or false accussing.but i messaged the guy to see the truth since they dont talk anymore,and the guy told me,hes sorry for lieing,but yes,she was with him,and it was a coverup,and shesno good.he said she said,he loves him,and wants to be with him forever.now i bring it upto her,this girl refuse to tell me the truth.i kept asking her,but she is not telling me the truth.but the matter is,i dont trust her at all,caue most of the time,we text 24/7,but while she was upthere,we barely text,and i would text her,sometimes she just text 1 letter words,or even even just ignore me the entire day,and all of a sudden,i realize shes back to texting me a lot,right when she finds out the guy dosent want anything to do with her.im very mad,cause i feel like the rebound guy,and she keeps using me to get what she wants,the guy told me shes no good,and it all seems so familiar.cause,the more i try to release,shes not letting go of me,i tell her bye,but she comes right back and text me,but shes not telling me the truth,she even sweared,but in texing and i definetly dont trust tht,at all!!!im freakin pissed,cause everytime i tell her to prove it,she insteads tells me,she dont have no time to argue,itslike she refuses to tell me thetruth,i even told her,if she tell me,illrespect her much more,but she still dosent tell me nothing.she very deceitful,and i dont trust her,wht do i do?the way she responds,its like shes a rebounder