Can someone help me how can i get through this

Hi im 20 years old and this is what happend to me. So i was in a relationship for 7 months. It was all pink. Everything was fine until 3 days ago. He started acting different. So i said to him to talk to me about his problem. Out of nowhere he started crying telling me that he doesnt want to loose me but he had feelings for his ex that he would try to get her back again. I felt miserable at that moment. We broke up. Immediately after we broke up he had followed her on insta and she had followed him. I wonder where all the i love you i miss you i want to see you went. I wonder what happend. I cant eat sleep nor keep with my normal activities i cant focus on anything i have this feeling in my heart i have panic attacks my heart beats fast when i think of him passing by me with her or even knowing that he succeed with her as he said to me in the eyes. Can someone please tell how to get through this.