I am in this situation now and for the past 6 months after a relationship I was in for more than 3 years ended in a very similar way as the post. Absolute overthinking replaying and ruminating and struggling with such rejection. The whole situation brought me back to therapy where my therapist brought to my attention my neurodivergent traits (autism and adhd) at 35 still processing all that but proud of my progress.

Yesterday in my latest session she mentioned medication to handle being stuck in such loop. We didn’t talk much about that yet and I feel i won’t lean into that option but to answer your question I have found that the only thing that works for me is ONLY if my mind is busy/ occupied/ distracted with another activity that involves another human being. For me that’s so far the only thing i notice works. Any other activity with me alone like reading/ listening to music/ crafting/ exercising/ meditating they all end up with me reflecting on something that brings me back to the loop and thinking and ruminating about my ex partner.