Ukraine can't achieve a battlefield victory but with foreign military aid they might be able to slow down the Russian advance long enough for the Russian economy to collapse. Russia is only able to keep their economy somewhat functional through fossil fuel exports. We can give Ukraine long-range missiles/drones and targeting assistance to wreck Russia's key export infrastructure (ports, pipelines, refineries, storage depots). And we can choke off much of Russia's shipping through sanctions and interference with their tanker "shadow fleet". There are also still billions in assets owned by Russian government entities, corporations, and citizens but held in Western countries so we can just straight up steal it all and hand everything over to Ukraine. There are no guarantees but as long as Ukraine is willing to keep fighting it's worth a try.

Ukraine doesn’t necessarily need to retake its lost territories by force. They only need to keep bleeding Russia out until the war becomes untenable for them. Russia loses like 1500+ soldiers and 100+ vehicles per day, just to move a line on a map a few meters. Their economy is hitting double digit inflation despite a 21% interest rate. Their recruitment efforts are running dry and they may have to resort to mobilization soon to continue to fill their ranks with more cannon fodder.

At a certain point they may have to come to the negotiating table, and the longer this drags out the less favorable their position will be.