I think he probably has realised that what Brexit means, is the UK leaving the EU, with or without a deal, that's why he's panicking now. Tusk has believed in Tony Blair and had hoped that Remain would gain the upper hand and negotiate a soft Brexit or Brino, or even a second referendum that would overturn the first one, but now he can see that hope fading rapidly.
Like his other colleagues leading the EU, Tusk is now between a rock and hard place, the Irish want their back stop and the British don't, if anyone's staring hell in the face its Donald Tusk, bridging the gap between Ireland and Britain has been attempted for the last 300 years without success, somebody should have told him!
I still think May will concede and call off Brexit altogether, then walk off into the sunset like Cameron and the world will return to normal, but with British politics changed forever.