I agree that @Sexnotgender has asked the most important question:
How does Russia get out of this?
If they can't achieve decisive victory, how do they withdraw or de-escalate?
And this is the problem, it's hard to see a way that they could do so.
It's possible that they could decide to be happy with getting their land bridge to Crimea -- of connecting Russia to the separatist republics to Crimea, along the Black Sea Coast. Sadly it looks they might get this. So perhaps they settle for that and withdraw.
But I'm not sure they would. Putin set out a very maximalist agenda. A land bridge to Crimea won't cut it.