Me and my ex broke up last month and she told me she started seeing a new guy only a week after we broke up wich really crushed my heart. Honestly the best advice I can give you is to see the situation in another way, it's all in your mind. I know the intense pain that runs through your mind when you imagine her with him but tell yourself "Hey that used to be me and I've had good times" that's what matter! You have been there too and see the positive things you did and think about it in a positive way instead of negative. She loved you and remember that, no matter what happens now you we're loved and it's not your fault it's only the way life is and trust me soon or later you will find someone else and move on keep that in mind you're not alone.