Ukraine currently is claiming that a victory would mean that Crimea, Donbass and Luhansk are all liberated. But is that actually required? They’re fighting an existential war, where a complete loss would be that Ukraine stops existing as a separate entity, or at least as an independent entity.
So perhaps a peace agreement where they lose significant amounts of territory would be still be a “victory” for them. It certainly was for Finland in 1944. You can compare what happened in Estonia before their independence from Soviet Union to see how that is true.
Also, Vietnam. War of attrition there was able to completely counter USA, a much more powerful military power. Although admittedly not a regional military power.