Sex File: I'm happy in my new relationship, why am I thinking about my ex?

It's ironic that trying not to think about something or someone almost guarantees that you will do precisely the opposite

Ever since I saw my ex at a recent event I have found him cropping up in my thoughts, occasionally sexually

I'm in a happy new relationship, but ever since I saw my ex at a recent event I have found him cropping up in my thoughts, occasionally sexually. 

Things ended messily and I definitely don't want to get back with him. How do I stop the past from impinging on my future?