Is it normal to deserve that ?

I've been dating my gf for more than 8 months and things are getting serious to the point that we often talk about marriage and such,we still go to college and we intended to get engaged in 2 years. Being curious is one of my vices, one day I decided to check her sincerity by creating a virtual twitter acct and see how far it could go and trust me guys i'm pro on that! I ended up by being disgusted,disappointed shocked! She started lying to me on the phone by telling me she went to bed at x time though she stayed chatting with "my virtual acct" til 7,8 am! Started telling some crazy stuff about me such I'm not her perfect match, there was no cruch and more shocking statements I don't want to repeat :(( and the icing on the cake is...she had sex with him online! No webcam but just "dirty" words and,when I've had enough reading the convers history every time I decided to tell her somehow I know about her story with "the virtuL acct" I didn't tell it was virtual, and now I forgave her but I don't see myself getting engaged with a such girl,and trust me folk it hurts...she begs me to forget about everything and cries all the time and tells me she doesn't eat study nothing 'cause she feels bad after all what she's done to me (I've been loyal to her and never thought about "cheating" anyhow) so now she wants me back...I don't know what to do...any advice?