Everyone is going to string you up for keylogging and spying, I however won't.
You suspected she was cheating, and you un-judgementally found out she was.
In your opinion.
Have you asked her if she thinks online flirting is cheating?
Man this is probably not going to work now but you should have told her, shown her the proof, dumped her & then left her. If she did care about you she would come back.
You got 2 options. Get her back or don't.
I'd go for don't, or atleast try to. Get her back (temporarly), build her emotions up & just leave one day. Make her know you won't take the shit anylonger.
Try other girls one might make you happy, after you have 10 others do you think you'll care so much about this one?
If you still do. (unlikely) you have some really special feelings and then you can consider getting her back again.