first of all, tell your girlfriend now to get over herself. you had a "cyber buddy" you never met this girl right? so why is your gf getting all upset? also why was she going through your belongings anyways? personal accounts are just that personal.
your gf has nothing to worry about. you were talking online and flirting with someone who could of or couldnt of been a girl. youre all about "doubts" arent you considering you dont believe me?
i think shes bugging out and acting immaturely. thats my personal opinion. you didnt do anything wrong. you talked to some girl online and said some things. big whoop. its not like she caught you with someone in the neighborhood and someone she knew/knew about or heard of.
again, what you did is what EVERY GUY/GIRL does. except they pay for it through special websites.
i wouldnt stress over it or beg for her forgiveness. i think shes being childish and if you DO have problems that this girl was able to fill the void with then you need to talk to your gf about it. if your heart is "straying" she needs to know.