The real miracle of the Winter War was the fact that Finland didin't crack at the very end. Finns had already spent most of their military resources in the static defence of their main line, the position that the foreign journalists rather ambitiously nicknamed Mannerheim Line - this "Maginot" had less concrete than the Finlandia House in Helsinki and was basically just a trenchline reinforced with some AT obstacles here and there, accompanied by ~hundred concrete emplacements at the whole Isthmus.
After Timoshenko had massed hundreds of tanks and thousands of fresh troops to Summa area and meticulous Soviet reconnaissance had identified the Finnish positions in the intented breakthrough zone, the war in the Isthmus truly turned into a battle of attrition.
Finns were able to disengage and slip away their frontline troops to their new line of defense further north - but it was a line on maps, hastily dug in by the same troops that had already been worn out in the battles at the main line. Unsurprisingly this line was soon breached as well, and the Finns withdrew to the last area where they hoped to stop the Red Army before it would be able to spread out from the narrow bottleneck of Isthmus and enter to the better roads and wider operational areas of Southeastern Finland.
Finnish HQ was really stressed by the time the truce negotiations in Moscow begun. Mannerheim insisted that the current positions should be held at any cost to deter futher Soviet demands, while Airo and Heinrichs, the commander of the troops in the Karelian Isthmus, stated that with the current rate of thousand casualties per week the frontline could perhaps be maintained for a week - after that everything would shatter and fall apart.
There were no operational reserves left, and even field replacements were hard to come by by February, as conscripts were sent to the front with minimal training. Chronic shortages of artillery ammunition had plagued Finns since the beginning of the war, but by now Mannerheim's orders to fire 50% of remaining artillery munitions to fool the Soviets into thinking that Finns were beginning to receive more aid from the Entente resulted to situations where batteries firing against the Soviet bridgehead at Häränpäänniemi fired a salvo of 2 shells.
But the Soviet military intelligence didin't know any of this. What they knew was that the British and French media was eagerly lobbying for military intervention, the Soviet Union had already been expelled from the League of Nations, and that the Entente powers were pressuring Swedes and Norwegians to grant military access to Allied expeditionary forces.
Most of all they knew that the operational season in northern Fennoscandia was turning towards the springtime. Back in the Karelian Isthmus the same spring thaw that would stop all large-scale operations in the Eastern Front in the coming years was locally known as rospuutto - but to the Soviet planners it was the same familiar rasputitsa that threatened to wreck their already stressed supply system in the Isthmus. With hundreds of thousands of soldiers and thousands of tanks and artillery pieces crammed to the narrow ~50km wide area between the western shores of River Vuoksi and the Gulf of Finland, the Red Army was in the danger of outrunning their clogged supply routes, thus preventing a quick new breakthrough that would have turned the war from field battles into asymmetrical and bitter guerrilla struggle.
So, Finns were desperate enough to accept the draconian terms dictated by Stalin, while the Great Generalissimus in turn wanted to take a timeout from a conflict that had been intented to be a short showcase of the new power of the reformed RKKA. Finns had made the Soviet Union look weak, and Stalin hated that. He was also cautious enough to cut his losses for now in order to see how the showdown between Hitler and the Allies would turn out in the Western Front in summer 1940.
In the end leaders on both sides felt cheated and bitter of the outcome of the conflict, and knew that this was indeed only an interim peace.