This is the problem of combat fatigue. Here's an excerpt:
Russia has suffered heavy personnel losses in Ukraine and is considering mobilizing about 450,000 more civilians into the military. This is needed to prevent front line units from wasting because of casualties, desertions and illness....Russian troops have already been on the front for over a year, and if combat fatigue really is setting in it should soon be obvious as the Ukrainians continue to pressure the defensive lines the Russians have established.
Russian units are not receiving enough replacements to make up for combat losses.
Captured Russian soldiers confirm the growing lack of replacements and the extremely long periods Russian units stay in the combat zone without any relief by fresh units.
To maintain morale and combat capabilities it is customary for both sides to withdraw infantry units from the fighting for some rest, home leave, retraining and integrating new recruits into the unit. Without this downtime, which can last a few weeks to more than a month, combat units decline in per capita effectiveness.
This happens despite increasing combat experience because too much time in the combat zone creates exhaustion and hurts morale, always to the point of sharply reducing combat effectiveness. The troops refer to the latter as combat fatigue or burnout.
Most troops begin to suffer from combat fatigue after various periods, with generally an irrecoverable breakdown after 200-300 days total in combat.
Taking troops out of a combat zone for rest and then putting them back into a less active combat area for a period (rotation) is the only way to deal with this. The United States developed other methods to keep career combat non-commissioned officers (NCOs) effective after the 200-300 day limits during the war on terror, involving more frequent and longer periods of time off, and with special treatment when off.
Both sides are suffering from combat fatigue but it is worse for the Russians, most of whose combat personnel have not been rotated (ever), and who suffer from corruption in the military, especially among combat officers who cannot pass up opportunities or enrich themselves at the expense of the troops as Russian officers commonly do.
The Ukrainians have declared they will continue to push against the Russian defenses into the winter. If Russian troops are forced to spend the winter in bitter cold, wet trenches with no hope of relief their willingness to resist will be severely tested.
It would be different, of course, if they were defending their motherland and their families, but in this war they are the aggressors and they have to be asking themselves why they should be suffering in what will likely seem to them to be a pointless and futile war.