Well, if they're democratic they should let the people (in which case my self-insert would apply) decide what to do. But a reasonable government could, for example, negotiate an immediate ceasefire and acquiesce to the winning side's demands to prevent further bloodshed. Or it could grab teenagers off the street and tentative migrants and send them to their deaths. Obviously, I already know that the Ukrainian government and their NATO allies prefer the latter, but we shouldn't pretend there weren't other options.

Edit:

What would you do if you were their absolute ruler? Surrender?

Well, yes? Assuming I give even half a fuck about people's lives, I'd prefer a negotiated peace instead of this shitshow any day. Russia's demands aren't even that hard to meet, they're just dragging this out for profit at this point.