When I was really ill late last year with depression (which I have been most of the time for a long time) I could not get repetitive thoughts about my former oldest friend dumping me for being ill out of my head. Even though I was on holiday with my bf and wanting to have a good time, all I could actually think about was fantasies about killing or wounding her, which was my response to the pain and let down of how she treated me when I was ill. It was where my head was at - she is the only person I have thought of in that way in my life - I have had no other murderous fantasies.
The only thing that stopped this thought going round is when I have had some substance that puts me in a better frame of mind. I'm guessing time might have helped too, but I don't know. In a better frame of mind, I think good riddance to her. When ruminating and depressed, my mind just keeps coming back to it.
At the moment, I am 3 days into Selegiline and it is helping me, so right now my frame of mind is better than it was. Also I found Modafinil really helpful - and am now told if combined with Memantine, you can avoid tolerance to Modafinil. So for me it seems I need something that helps Dopamine.
I also read that inositol can be good for repetitive thinking (it can help OCD for example). I haven't tried it properly but it is a natural sugar and available OTC - and the dosage and research for it is online.
What I mean to say is, I have learned that so many of my thought patterns, if not all of them, are a product of my brain chemistry. It's very strange for example, eg when I am happier on medication, my dreams change. The past two days or so my dreams have been happier and more normal. In the past, they were all about quests and deeper meaning - because I think my brain was trying desperately to try and give meaning to my life, as the days were so torturous. So while you think you cannot forget this person, the reason for this is your brain chemistry/brain patterns.
The other thing that might help - it was of great help to me for many years - but can be a double-edged sword and I don't know if it's safe for you, is weed. I used edibles which did make me hallucinate, but put me in a better frame of mind and in some ways have made me into a better person now I am off them.