To be honest, I think the fact of her cheating on you so long ago wouldve been eating at her for ages (hence why she told you now). Although I dont condone this sort of behaviour at all, I believe that she must have feelings for you if she was to tell you that she cheated, and so long ago. I still wouldnt take it lightly, give her some time to think about what she has done. If you feel the relationship has nothing left then leave her but if you still love her then you should consider staying with her. Good luck.