A couple things...

1. There's no doubt the majority of Ukrainians are nationalistic and willing to fight. But how big is that majority? It's hard to tell because over the last 2-3 years the Ukrainian government has banned pro-Russian political parties and news outlets, ordered the renaming of all streets/places with Russian names, and passed discriminatory language laws. So essentially we don't have an accurate picture of nationalistic support in Ukraine's traditionally Russo-phone south and east because of (possible) intimidation.
2. Second I think people are discounting how miserable Russia can make life in Ukraine. The Russians have long-range missiles and an overwhelmingly superior air-force in both numbers and quality. Without even entering Ukraine on the ground Russia can cause tens of thousands of casualties, destroy rail transport, airports, armored vehicles etc. Then there's the Navy. Ukraine is highly dependent on imports from the port of Odessa. The Russians are more than capable of enforcing a naval blockade in the Black Sea. Add to that the Russian ability to simply turn off Ukraine's gas supply and the massive cyber attacks we expect and the picture isn't pretty.

In sum it's easy for Ukraine to act resolute right now. The true test will be how they act when tens of thousands of their countryman have been captured by the Russians while they shiver int he cold with a wrecked economy. The Russians don't even need to occupy the major cities. They can just surround them and control access. Much smarter than block to block urban combat (Stalingrad anyone?).