Why did my ex move on so quickly?

What I suspect you really want to know, is one or more specific questions.

I know from my own “misadventures” in love, that my ex’s were able to move on quickly because they were more attractive than I am, and far less shy, and sometimes because they were far less picky than I am.

or if they moved on quickly because they were secretly starting up with their current person before you broke up;

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and so on.

Such as, if your ex moved on fast because they never actually loved you;

or if they moved on fast because they can control their emotions like a simple light bulb;

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Different tack: I don’t think you asked what you want to know, you asked what you’ve heard other people ask.

Of the ones I was able to talk to about it all rationally later, some of them admitted they “used” a new “hook up” the same way some people use drugs, to dull the upset over our breakup; some said (perhaps as revenge) that I was easy to replace; some said they just got lucky. Most didn’t stay with the quick replacement, but a few even “panic married” the replacement, as part of being upset about the breakup.