Defenders are more likely to win longer wars. So, the idea that Ukraine can’t win because the war is going on so long, is backwards. The U.S. revolution was an eight year war.
The Americans were largely getting their asses kicked and were dependent on foreign aid, but in the end, they won.
The aggressor usually sustains more casualties compared to the defender. The defender often gains resolve the longer the war lasts, while the invader questions why they are doing this in the first place. The defender is fighting for their homes, their territory, and their independence, while the invader is fighting to gain something.