From my understanding, Russia is running out of gas and their economy suffering significantly.
Ukraine is getting stronger and better defended. If they run out of money, they may be in some trouble, but that doesn't seem to be looking likely anytime soon (large influx of EU money coming soon).
Ukraine can trade small amounts of land to allow Russia to grind its self down (Pyrrhic victory after Pyrrhic victory). Eventually Russia won't be able to sustain their momentum and can be pushed back then (already seeing some evidence of this beginning).
Edit: spelling