My girlfriend cheated on me and lied to me for 5 years about it. What should I do?

First of all hello ladies. I'm just gonna start by saying thank you for any comments or help in advance. I know I'm just gonna sound like a real creep or something but I can assure you I'm not now with that out of the way let us begin...

Me and my Girlfriend haven been dating a little bit over 5 years and I recently have been thinking about settling down and getting married. Everything was fine until my birthday. I go out drinking with our friends and her, everyone leaves early and goes home but me and one of her friends (lets call her Z). Z lives a block behind my house and me and her have become close friends through the years so taking her home is no night deal. On the car ride home in a drunken haze Z confesses to e that my Girlfriend cheated on me over 4 years ago and could not keep that secret anymore "your a great f***ing Guy and you don't deserve to be lied to for all these years". Obviously I was in total disgust that (A) she cheated on me and lied to me for over 4 years about it and (B) that her two best friends knew about it the whole time...her best friends are Z And O...I confronted all three of them my Gf, Z, And O the following day only the finally realize it was all girlfriend broke down into tears and gave the same typical "got caught cheating boyfriend routine" I'm sorry blah blah blah I love you blah blah my stupidity I forgave her...but I never forgot what I felt that September it will be my one year anniversary of my heart being broken and flushed down the toilet...I have never done anything to anyone to ever deserve this...I'm a 29 Guy who strongly believes in to make matters even better her friend O has been making strong sexual passes at before you all judge me..let me say this very clear...I have NEVER CHEATED IN ANY RELATIONSHIP...EVER...I am as loyal as they come and will be stick with you through everything but this is a new alley way for I'm not gonna sit her and tell you that her best friend O isn't hot...she's a knockout in my eyes but then again I'm a ugly fat ass...either way I've got a tough decision to make should I sleep with O as payback for what my Grrrr did to me and if so how do you think O will react?
My girlfriend cheated on me and lied to me for 5 years about it. What should I do?
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