Because the break up was mutual it's possible you really allowed yourself to move on and only now as someone new is on the scene you feel differently. I feel strongly that you should allow yourself to feel upset for a period of time as it clearly needs to come out of your system. Try to focus on other things and be happy that she is happy. No one can plan when they are going to like someone new, things just happen, it may only be temporary. Please dont make her feel bad for this. try to keep your communication and visits as normal as possible obviously allowing her time to live her life too. I am great friends with a few ex's some I have needed space apart from, some I havent, Bad or mutual break ups, but time is a healer and if your friendship beyond the relationship is meant to be then it will be there. :o)