To also be absolutely clear, no Ukraine is not fighting for its existence. Russia's interest in territorial gain extends only to the areas of majority Russian-speaking, ethnically Russian populations.

This is for the very simple reason that it's literally not possible, in the modern world, to hold territory in which the native population does not want you to remain. Trying to do so will only result in a very long, very costly anti-insurgency campaign and your eventual defeat. This is exactly what NATO was hoping for when they baited Russia into attacking, that they would try to occupy and hold the whole of Ukraine. Instead, Russia only bit off as much as it could chew and is now allowing the Ukrainians to waste all their soldiers and equipment by attacking into impenetrable fortifications. When that's done, they'll counter-counterattack, grab some more Russian speaking land, and then let the limping Kiev rump state collapse under neoliberal austerity shock doctrine privatization administered by their 'allies' in the West. Ukraine could have avoided all this by upholding either of the two Minsk agreements that were painstakingly worked out by diplomats from both sides, but unfortunately the hand of their masters in Washington was too strong.