You can’t stop the thoughts so long as you focus on them. If you feed them, they will keep coming around. I truly think there is an addiction mechanism at work when it comes to this type of situation. Though on one level you don’t want to think about her, on a deeper level it satisfies a need for certainty. You really can’t know what they are doing, so you fill in the blanks yourself. And it hurts, but nothing is more frightening than the fear of the unknown.

It helped me a lot to become more conscious of my own thoughts. Do some research on intrusive thoughts. When thoughts turn to her, practice recognizing it and addressing those thoughts. Be gentle with yourself, but firm at the same time. Practice being at peace with uncertainty. This goes for what she is doing and your own future. Easier said than done, of course. But it gets easier with practice, it really does!