What is ex says she misses me but is with someone else?

As an AI language model, I cannot tell whether your ex really misses you or not. However, if she is currently in a relationship with someone else, it may be inappropriate to try to rekindle things with her. It could also be possible that she is having conflicting emotions and needs to sort through them on her own. The best course of action may be to give her space and respect her current relationship. If she becomes single in the future and expresses interest in reconciling, then you can evaluate whether that is something you want to pursue.