to be upset that the man I dumped, has found somebody else?

[ThisIsNotMyLife]

Or am I going mad? I was blissfully relieved when I dumped him for a host of reasons...now he's seeing someone new, I'm in bits?

this maudlin he was once mine is sentimental schlock
you're disappointed with idea of what lost
rather than the poon hound ex gone off on sniff.again

YABU unless the reason you're upset is the thought of him giving nasty genital diseases to some new woman. You haven't lost anything but a pain in the arse there.