You probably didn't start the day thinking about your ex, but you might talk to an old friend who knows your ex, which could lead to you and this friend talking about "old times."
Whenever you have a fight or disagreement with your current partner, you tend to think about your ex. You did this as expected, and your partner even knows about you.
During the transit of Mercury square Lilith, the ugly truth will come out for you today, Sagittarius. You will realize that the only reason you keep thinking about your ex is because you are alone right now.