Most Helpful Girl
You still like her &you just remembered her that is it.
Most Helpful Guy
I've been in your situation and what worked for me in the past is to clearly define the reasons she is your ex beforehand so you can recall those reasons at a moment notice to in a moment of weakness. Etch those reasons into your mind and use them in to control any chance encounters you might have with her.
Once you have yourself under control you can approach those moments from position of strength and confidence. Furthermore, there is a chance she might pick up on that new confidence and might find it appealing. You can have some fun with that and a part of her will secretly love it. ;)