Why does it hurt so much when your ex moves on (even when they "downgrade"), and how did you overcome it on your own, without moving on?

I am sorry if I sound out of line or rude, but why is it known to hurt so much when your ex (example, boyfriend) moves on, even if the girl is nowhere near as attractive, nice or successful as you are? Is it the thought of being compared/replaced? I've personally never really felt jealous of or afraid of any of the girls that my exes moved on/cheated with, but then again I do not date "attractive" guys; just average joes that use me as a trophy, I guess. I do have low self-esteem, though. Aside from moving on yourself, why does this feel horrible? The thought of this kills me, but it is inevitable and honestly fair game.
Why does it hurt so much when your ex moves on (even when they "downgrade"), and how did you overcome it on your own, without moving on?
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