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.
Afghanistan was close to dead last in terms of GDP per capita when the Soviets invaded. (Some things never change). It hadn't fought a real war in over 50 years. Its finances were a shambles. Thanks to a coalition of anti-Communist states, they got a few billion dollars worth of imported arms and money to feed their guerrilla force, and proceeded to bog the Russians down in an increasingly demoralizing war.
In contrast, Ukraine is roughly in the middle of the pack, income-wise. If we hand a few billion dollars a year in food and weaponry to the Ukrainians to ward off the Russian invasion, I expect they will give at least as good an account of themselves as the Afghans did. After WWII, Stepan Bandera and his people fought the Red Army for over a decade with zero foreign aid, and were destroyed only after the Red Army slaughtered hundreds of thousands of civilians with them - which is presumably why Khrushchev gave the Crimea to Ukraine - as a consolation prize for mass killings of Ukrainians. If today's Ukrainians have anything like Bandera's fighting spirit, a few billion dollars in equipment and food should see them pushing the Russians out of the country altogether.