The relationship is over. There is no point in wallowing in misery over what happened in the past. Any urge to confront the person with the knowledge of their infidelity is an expression of the desire to have control over that person, which is meaningless now. Let go of her, and get on with your life. Be glad that you aren't with her anymore. It would have been much worse to have stayed with her and then find out that she was cheating on you. Be glad that your cheating ex-girlfriend is your ex-girlfriend, and not your wife.