In my opinion you should grieve. You didn’t lose someone in the traditional way that causes grief, but you lost something significant and in a really terrible way. Worse than that, you lost trust for someone you cared for. They disrespected you. I think you should allow yourself to wallow, watch a sad movie if that comforts you and do whatever the hell makes existing a little bit easier for a few days. Cry, or go punch a kickboxing bag at a gym, express yourself. Meditate about it, journal about it. Write out all your horrible feelings and the things you’ll miss. Then, maybe after a few days when it gets a little more numb and a little less painful. Try to put your trust in the future. The past is over, only now exists and time heals all wounds. Take each day a step at a time, notice the things you have that make you happy and trust that one day you’ll have something else that brings you joy and this will be just a bad memory one day. She wasn’t honest with you, or herself, and that’s not your fault. People who cheat are deeply damaged. I’m sorry this happened to you, but at least you won’t spend anymore time with someone so selfish.