What do you do once you learn that you've been cheated on 3 times, how do you proceed?

Lets be reasonable, Everyone falls in love and falls out of love, i met this girl and its currently been 1 year, something special and yeah i'll admit its my 1st relationship a real one. now let me tell you a story:

She was off saying goodbye to a friend who was going to the navy last year and it was a party of hers my girlfriend attended and pretty much met this guy whom i shall call Guy A, so my gf and this "Guy A" decide to talk and somehow happens to sweet talk her, she had no alcohol she was 100% sober and one day drives over to her place and pretty much has intercourse she tells me everything and i was pretty heart broken by it, i didn't know something like this could even happen but at first she let on small details like just a you know a "BJ" and that was it but as the days went by she admitted more to it and it pained me so I even got around the bend to forgive her and tell her i love her, i did and also i admitted how much she meant to me, that i wanted to spend my life with her as a married couple after all its not the guilty part of me that wanted to stay, it probably was because i was a fool because i gave her a second chance that she would be faithful and that would be the end of it, I truly thought that was it but it wasn't the end of the story she told me about how she texted back an forth with this Guy A and pretty much by today told me that she confessed to doing it again 2 more times with the same guy and the last time she did it was the 2nd week of April (This month) only she felt far more guilty and wanted me to tell her off,

Of course i felt so freaking mad and upset i just couldn't handle myself. i don't know how someone can handle this amount of pressure in learning to live and learn this out the hard way she even told two of my friends behind my back and telling me this so late, Is ignorance truly bliss? i best cannot say how to answer that without ever actually acknowledging how this Guy A may have performed too. it put me down to shame again using the word learning every single detail because i had the right to know.

How does one get over something like this how does one forgive and forget especially if it was my first? i know alot of what people said because they had suggestions written up on the Yahoo Answers and i could somewhat relate but everyone has their own story to tell... does anyone have an answer to my question? i mean i've heard the dump her and find someone more respectable who will love you, the whole cheat on her back method, i feel like i barely know myself because of how much time i put into someone like her and so please feel free to express yourself and respond as you see fit