Post breakup obsession

1(28M) was broke up around 10 month ago. It was painful. We belong to the same group of friends and the relationship (1.5 years) was meaningful. I started psychotherapy (still going on), moved out my parents house, invested in hobbies, did some local trips as I had planned, tried to work on my friends and family relationships and things were slowly improving. But since 2 month ago (after discovering she returned to her ex) I kind of went to depression and feeling super bad. I think I started again to being obsessed with her again and kind of being in the initial state of the post breakup. I know why we breakup, I also have my responsibilities and I understand she didn't want to continue. But she moved on (personally and professionally) and even I took steps to myself, I feel that I'm not progressing and feeling better as I should. Any tips or perspectives that can help me?