My gf cheated on me...!?

We have been dating for almost 3 months now... our relationship has been unstable lately due to some small problems that my gf make big deal of... Soo last night me and my gf was talking at msn then there was this random guy went intio a convo with all 3 of us... then a nightmare happened... My gf told me it's over... then basically she said that the guy is his bf.... then i was devastated... then this guy and my gf put alot f **** into me... like everything.... and i didn't know what to do... Im seriously in love with this girl so i stayed up all night to write a letter to her saying im sorry for everything and told her that she means everything to me blah blah blah.... then gave her the letter first thing in the morning with a bouquet of flowers lying down her doorstep...then when i got home she talked to me and told me she forgave me and everything and i dunno were back together...? then it gets complicated.... first i was shocked about everything...! then first thing i did was to say thank u and appreciate her acceptance of me then i sarted asking her about the guy... this guy was from omeggle where u can talk to random ppl around the world... she met him last month... and she told me that this guy was there for her when we 2 were fighting and he guy ask her out then she said yess... while she was dating me still but in a instable relationship but we were still seing eachother.... sooo basically she cheated on me right..? and i had a bad past f getting cheated on and she knew that... and i just dunno if i can forget this horible experience... but i really love her soo ill take everything... but i just need an opinion if what i did was right or wrong... and what should i do and not do....