Why did my ex move on so quickly?

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

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

and so on.

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

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.

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

Why do a lot of women have a crush on my boyfriend when they know he is in a relationship with me? I am starting to feel insecure too. What should I do?

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

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.