Homemade taffy turned out too thick and hard
Temperature is certainly important. If you start out with a hard candy straight from the pot you're gonna be largely out of luck regardless.

But really, the signature taffy texture comes from the continual pulling, folding, pulling that happens after you remove it from the heat. Pulling taffy works lots of air into it and stretches out the strands of sugar crystals into thinner, chewier shapes. So be sure you really work that taffy once you take it off the heat (and before cutting it).