I think the best way to react to seeing your ex in public or if you're in an environment where you have to converse with them is just talk to them like you don't even care that they're there because most of the time, especially guys do this, is that they'll just pretend that their ex doesn't exist even though you both know that you are in each other's company and you're actually telling that person that you do care about them being in your environment because you're going out of your way to ignore them when you both know that you used to date it's like fight, flight, or freeze don't be hostile, don't ignore them, especially if you have to converse with them and talk to them and interact with them just act like they're another human being and not necessarily a friend but you can act civil and that speaks so much more than just obvious silence, ignoring, hiding or hostileness, anger, aggressiveness just being civil I think that hurts a lot more because it really shows that you're not affected by their presence