What you are experiencing is perfectly normal. It's natural to be curious and wonder what the ex is doing etc. Initially you can't help yourself, but in time you simply won't do it anymore. If it bothers you, then you need to make a conscious effort NOT to look, and to try and force your thoughts away from her if they start. Once that thought process begins,....distract yourself with something else. Again, in time you won't need to force yourself because your thoughts will rarely go there.
Fake it till you make it