Really serious problem with girlfriend?

I've been through the same thing and did exactly what you were going to do. There are women out there who are always looking for that fairy tale life and don't figure out what they have now is the thing there looking for. First don't sure her your emotions because she really won't care about it and will try and say anything to defend herself. Just go straight up to her and ask her if she has feelings for her ex. Don't tell her how you know and how you feel about it or how you figured it out. It's time to act like adults and not do any of that high school drama stuff. If she denies it then just walk away and don't say anything. Yelling at her and say bad words will not solve anything. Just give her cold shoulder. Silence will be the worst thing that she will ever go through. Because with silence she won't be able to make up excuses for herself and won't be able to wiggle her way out of this situation by trying to throw it back at you. do not talk to her or have any contact. Give it some time like 2 weeks or even more until she comes out and tells the truth. If she does not tell the truth and doesn't try to contact you then she really doesn't love you and I would advice you to leave that relationship. If she does come and tell you after the two weeks then its your decision to see if you really love her and if you can make it work. That's all i have to say. Just don't Blow up on her or tell her how you got the information because you will be putting the ball in her court then. Just walk and show no emotion. Don't be sad or happy. Give a blank face so she doesn't know how you feel. If she loves you she will tell the truth and take full responsibility for her mistake and then it's up to you if you want her in your life... GOOD LUCK BRO. JUST know that there are many other women in the world that will show you true love