I suspect their pain threshold is not what it used to be. The Soviets cried uncle after losing 13K dead in Afghanistan. If the Ukrainians are prepared to take the pain of inflicting tens of thousands of dead on the Russians, odds are that the Russians will pull back. It depends on what the Ukrainians are prepared to sacrifice.
I think you misread the situation. Ukraine is in NO shape to fight Belarus, let alone Russia. The country is completely bankrupt. Putin could shut down all the gas lines leading into the country in a heartbeat. Ukraine’s military is in disarray after the coup, with many groups being charged in court over the protest killings.
If these people are indeed Russian commandos rather than just separatist rebels (and I’m not convinced they are), then there is nothing Ukraine can do. Her hands are tied.
The Ukraine is not Afghanistan nor are the Ukrainians, Afghanis. The language and cultural issue is deep-rooted and will be a running sore and distraction.