i can’t stop missing my ex after months

My ex and I dated for a few years and then had a really tumultuous break up where we were seeing each other on and off for a while afterwards. There was drama during that time and we both hurt each other in different ways. We’ve finally cut all contact (his decision in the end) and haven’t spoken in 4 months. When i’ve tried to reach out in a friendly way he’s been cold, and he is seeing someone new now who is definitely more compatible with.

I know we shouldn’t be together but I somehow still miss him and think about him constantly even after all these months. I am constantly having to force myself not to reach out again. I am blocked on social media so I don’t even stalk him but somehow still can’t get him off my mind. I don’t get it and i’m so tired of it. I feel like i’ve done everything - I talk to friends, started seeing someone new, journal, write letters that i don’t send, all of it - and somehow he still takes over my thoughts.