我們原來說過“耳朵蟲”（earworm），也就是經常在腦子里出現的歌曲旋律。今天說的這個“心蟲”跟“耳朵蟲”有相似的意思，只不過“心蟲”指的是讓你難以忘懷的一些感情。

Heartworm refers to a relationship or friendship that you can't get out of your head, which you thought had faded long ago but is still somehow alive and unfinished, like an abandoned campsite whose smoldering embers still have the power to start a forest fire.
“心蟲”指讓你難以忘懷的一段愛情或友情，你以為這段情早就過去了，但卻時常想起，難以釋懷。就像棄置的露營地里殘留的火星，仍然有可能引發一場森林大火。

For example:

I want to settle down with someone but this heartworm of my ex kept disturbing me.
我很想找個人安頓下來，但是跟前任的感情一直揮之不去。

（中國日報網英語點津 Helen）