I have been with my ex for 5 years now (2 in relationship, 3 as FWB). The reason our relationship ended was because he cheated on me and has been with the other girl since then. I couldn't bear the emotion toll and thought it was a good idea to stay FWB until our office contract ends. We work together. Renetly have started noticing that he doesn't care about me like the way I do. He treats me according to his own mood swings. (Great for a week and like shut for the next). No matter how much hard I try to get over it. I just don't. It hurts me every single time. Many guys approach me but I just don't seem to care about them. All I can see is him. Even his relationship with the other girl doesn't bother me. Any suggestions on how to get over it?

TL;DR: How do i stop getting hurt from my ex turned FWB actions?