A more or less promiscous boy/man/girl/woman falls for someone that is just as likely too sleep around... And thus waking up to the reality of sudden and constant heartbreak that they usually inflict upon others all the time.

I do like me some happy endings, though it is not a requirement that they end up together. But they should at least have some kind of longlasting relationship with someone in the end.

All genres are equally welcome, but I would prefer some comedy

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~4400 manga completed
~1700 ongoing...

Ore ga Doutei o Sutetara Shinu Ken ni Tsuite, it's not a comedy though.

You have just described Totally Captivated. It's a yaoi, but it fits what you described perfectly and I highly recommend it.

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Cynicism at its best.