Traumas
The soldiers in the trenches underwent many traumatic experiences. If you were a soldier in the battlefield, here are some of the many traumas you might endure.
Illnesses
As mentioned in the "Trenches" section of this site, sanitary conditions of the trenches was poor, and many soldiers became ill. In addition to illnesses unique to warfare, such as Trench Foot, Trench Mouth and Trench Fever, many soldiers became infected with dysentery, typhus, and cholera. Many soldiers suffered from parasites, and these caused further illness. Sometimes, a soldier would mistake a lethal illness for a more innocuous one— for example, mistaking Trench Fever for regular influenza— and take too long to seek treatment. Despite all the horror stories of gas attacks and gunfire, illness was the number one killer of soldiers during the Great War.
Wounds
It was extremely unlikely that a front-line soldier to survive the war without being injured, and most soldiers were injured more than once through the course of their service. Some soldiers received what was referred to by the British as a "blighty wound," a wound serious enough to be sent back home, but not serious enough to kill them. This fate was, however, uncommon, and soldiers had to receive medical services on the battlegrounds. At this point in time, medical services were relatively primitive, and many of today's life-saving antibiotics had not been discovered. Minor injuries could become lethal for a soldier, as it might become infected, or exacerbated by the enemy's gas.
Early on in the war, official truces were organized to recover the wounded and bury the dead. Although the commanders disapproved of this, soldiers understood that it was mutually benefitial for both sides of a battle. All men carried emergency field-dressings, and, when wounded, would attempt to treat their own wounds. At the unofficial cease-fires, the injured would wait until stretcher-bearers, consisting of two or four men with flags of the Red Cross, fetched them to their side's trenches for healing.
Shell Shock
In addition to the physical harm of war, soldiers often underwent psychological damage as well. Soldiers who endured constant bombardment would suffer a condition called "shell shock", what we would recognize today as post-traumatic stess disorder. This condition was not understood at the time, and symptoms included fatigue, irritability, headaches, and lack of concentration. Some soldiers suffered from mental breakdowns, and could not continue in the front lines. They were reduced to shivering wrecks, and, if they were officers, were sent home to recuperate. Ordinary soldiers, however, were not so lucky. Shell shock was not recognized as an official medical problem, and many high-ranking officials thought that the sufferers were cowards who wanted to get away from the fighting. Some sufferers refused to follow orders; others commited suicide or deserted. Punishment of these men was harsh, and ranged from court-martials to even execution.
Self-inflicted Wounds
As mentioned early, a "blighty wound" is a wound serious enough to warrant sending a soldier home. With the prospects of being killed or disabled in warfare, soldiers sometimes hoped for a blighty wound, and some even took the matter into their own hands. Some soldiers shot themselves in hopes of ending their service in the frontline. This was a capital offense, and, for some countries, a man might face execution for this crime. However, this was a risk many were still willing to take, and attempts at blighty wounds continued throughout the war.
In more extreme cases, some soldiers became unstable and were not able to handle the trenches. They killed themselves rather than continue on, or let themselves be killed by the enemy.
Death
Death was, unfortunately, a huge part of trench warfare. There were many ways to die: for example, illness, as mentioned before; the precisely-aimed bullet of a sniper; and the deadly poison gas used by both sides of the war. During World War I, around ten percent of all fighting soldiers died, and many more were wounded.
For More Information
I find that the best way to understand the traumatic experience of trench warfare is to watch films about it. Many films do a good job of depicting the misery and scale of the Trenches of World War One. Movies you should check out include Paths of Glory, All Quiet on the Western Front, A Very Long Engagement, Capitaine Conan, and Regeneration. Some of these movies are not appropriate for all audiences, so please do your research before renting or buying them.