In the instance of "Grass Is Always Greener" Syndrome, your ex is likely to have met someone fresh before your split, which may have precipitated the breakup. Also, your ex-girlfriend is in her early twenties and wants to keep her options open in order to discover what is available to her. She might be tempted by someone who appears more successful or richer than you. The fact that your ex is meeting other people doesn't mean that you are any less attractive or desirable than she is. In fact, it's just the opposite: You're more attractive because he's looking elsewhere. The more people your ex meets, the more opportunities there are for him to find "grass is actually greener" situations. The main thing is that you need to accept that your ex has moved on with her life and stopped caring about you. Even though you both decided to end things, she still feels guilty about this and feels like she should stay with you. This is not possible so don't try and force her to stay if she doesn't want to. If you really feel like you've reached an ending then focus on moving on with your life. Do something new with your time and stop dwelling on past relationships. If you do this then you'll see that "grass is always greener" syndrome isn't as big of a deal as it first seems.