"Maybe it was because Grant and Sherman had their personal demons driving them to a higher performance level that they succeeded where other Union leaders failed."

I think, under a general impression, the more common denominator was that the previous Union Generals, being professional soldiers, cared more for their troops than either Grant or Sheridan did. By the end of the war almost none of Grant's Generals, including Meade, would press home any of the attacks against Lee's entrechment around Richmond. There just seemed to be no desire to uselessly get killed by the Union soldiers after Cold Harbor.

There were plenty of oppertunities in late 1864 to break Lee's thin defenses but none happened. For some reason no one seemed to want to really press home the killing blow even under Grant's command.