I think of my ex-wife like an "other woman."
Why do you fantasize about your ex?
I don't know, but I get tired of her saying she'll find someone else.
Well an ex-spouse had a huge impact in your life, so it's normal to still think about them from time to time. But you still have to let them go eventually and find someone who will really make you happy, because if you keep dwelling on the past like this you'll only be unhappy and want to go back.
She broke up with me so I am still deep in love with her. I stalk her from time to time on social media.
You shouldn't obsess over an ex. If she was the one that broke up with you, you need to move on. If you really care about her, you'll let her find happiness with someone else while you find your own happiness too. Don't let her stop you from falling in love again.