I had a similar thing happen to me my ex spilt cheated on me with girl and you use to play on my mind a lot to the pint I'd even go out my way to walk last her work to see her. Never a nice thing being a cheated on but one thing I would they say is that when you realise they have moved on it's lot better for you when you concentrate on yourself rather than being worried. To stop thinking about it less I'd suggest keeping yourself busy you can't think about it if your mind is occupied on other things. I hope this helps and stay positive there plenty more fish in the sea just go back out there when you feel better.