One must spare a thought for the rescuers who had to trawl through it desperately looking for signs of life and how deeply this sort of thing must affect any human. I don't know how they can do it.
I don't know how it is handled in Spain. In Germany firefighters, paramedics, policemen and other rescuers learned (and frequently repeat) how to handle big crash or crime scenes and there is a general structure of how to organize themselves and their work.
So training is the first step not to feel helpless and lost, everyone knows his duties and has to concentrate on that.
The uniform provides some protection because it is not "you", the human beeing, it is the firemen, the paramedic, "your function" which takes part in that scene. And don't forget that no one fights alone, you are sourrounded by a team of other specialists.
Rescuers should be taken out of the scene early, "fresh" collegues should send in frequently. That was one of the biggest problems of 9/11 when NYFD could not change folks in time, they just lost so much firefighters...
Nevertheless there also is a lot of general education regarding psychic reactions before you start working as rescuer, everyone gets instructed once again after returning from the crash scene. Specialists take the rescuers immediately to a quiet place without public or media, but to eat and drink something, and to talk about what happens. Again everyone is instructed what could happen to him, what feelings, thoughts and dreams could occur. They were told that everything is a quite normal reaction to an absolut unnatural experience.
There are always psychologists who can be contacted 24/7 on the following days, everyone is invited not to hesitate to contact them, and some moore meetings for every rescue team take place in the followings days. People can talk about their thinking, how they feel, the "instructors" also check who needs special assistance.
From experience we know that some people, confronted with the most horrible scenes, do not develop any kind of problems, whereas other ones encounter heavy disturbance even if they had easy jobs obviously.