you can definitely steal it. if you are ripping from an "unpublished" song, then you're the only one who'll know
if you've already put the original song "out there", then you should consider from your fans' perspective what directly copying part of that song into something new will sound like. only you (and your fans) can really answer that - it depends on your style, genre, etc. but more than likely, it may be perceived as a gimmick to cover a, well, lack of creativity. i've never heard any artist that directly copied a part of their song into a new song unless it was some sort of live mash-up/medley, even though we could all probably name artists and bands whose hits seem like they took last year's single and put new words to it (see: nickelback).
from an artistic standpoint, we all would like to say "my art is about me", which gives us license to do whatever we want. but if it really, truly was just about you, none of the rest of us would ever know about it.
art is meant to be shared, to inspire others to more creativity. while there are definitely true artistic geniuses out there who are both recluses, yet so in touch with their own humanity that they can speak directly to our souls - you'll probably not find them cloning parts of their previous work either.
