Ukraine can't win because Russia is the superior army so a "draw" is their best bet and the best way to do it is guerilla warfare to force Russia to withdraw. We already know that Ukraine will suffer more losses unfortunately because as you said Ukraine doesnt have much air warfare capabilities and as we saw between Armenia and Azerbaijan, the army that rules the air, wins.Nah we gotta be careful of the opposite, fact us Ukraine is an infantry army with almost no air. Their only hope is to hole up in the cities and turn this into a grind. You cannot win in modern war without air power and with no long range artillery capability
Russia has closed off its airspace until early March so we can believe that they are expecting at least a couple of weeks of operations. Ukraine needs to make it a meat grinder for them.
The Russian population is not THAT much behind Putin, the economic conditions for the average are bad, the Ruble is crumbling and the world will isolate Russia economically (except for its gas smh) so the morale of its people will play a bigger and bigger part as long as this keeps on.
The longer it takes, the worst it becomes for Putin and most importantly for the Russian people that will suffer the economic consequences the most.