I beileve that "The heart that feels not now is dead" means that the heart that is no longer beating is dead. Or the person that contains the heart lost his/her feelings and is feeling dead.
Explanation:
I tried to break this out, so we know that it's about a heart. I was a little confused with this part but now I understand it "that feels not now". I believe that this means that it has no more feelings or it just isn't alive anoymore. You can really understand that the heart is dead or broken when it says "is dead." I hope this helps you!
