This might not be entirely what you sre looking for, or allowed, if so let me know.

I have done a fair bit of work on Holocaust rememberence, including visiting 2 of the Auchwitz camps, running events, general awareness stuff, through various charities, snd it is quite hard to deal with as your post suggests, but the way me and the people I worked with kept ourselves grounded (so to speak) was to focus on individual stories of people.

Stating its 6 million people, or what have you, is just a number to people. People tune out numbers or can't fully grasp just how many people that is because its an astronomical number people that we have never seen ourselves all at once.

If you say however, this is John, he lived in Krakov, he was 19 years old, he was a trainee plumber, had a girlfriend called Sally, who he lived with and supported as she raised their newborne son Eric. They plan to have a winter marriage as it is their favorite time of year. The marriage never came. The son never grew old enough to know his father. John never became a plumber.

Etc. Its a crude example, but I hope it conveys the idea. You can get invested in a personal story (Like Anne Frank, people get invested in her story), because one persons story is easier to connect with, compared to the story of the 6 million faceless people.

So with that in mind, it takes a bit of self will too, but this is what I and those I did work with did, and what we encouraged to do when doing rememberence work with the public.

Edit: any questions or further expansion I don't mind, might take some time to reply, currently abroad for a while with limited internet but I'll get there, promise :)

When I am "doing" history, so to speak, I am usually desensitized to the topic in that I can usually look more objectively. I study the way that Americans represent the Holocaust, and so I have done a fair bit of Holocaust reading. Yet, when I am in "work mode," I am analyzing, considering, etc.

That said, when I am watching a film or reading in my leisure time, I allow myself to get more emotionally involved. Usually, as u/leviethan pointed out, personal stories play a role in my connection and sensitivity to the topic.

Oddly, my studies converge in a way that is quite odd. I study US representations of the Holocaust, so sometimes I have two very different reactions to the same text. One Christmas I had to watch every English language film related to the Holocaust made between 1945 and 1990. I watched such films as Sophie's Choice and The Pawnbroker. Since I was doing this as research, I kept my mind focused on how the writer, director, and actors were portraying the Holocaust. This, usually, allowed me to avoid having an emotional crash. However, if I watch such films and let my mind go with the film instead of analyzing it, I am much more emotionally involved.

Hence, I suppose the answer is that I am thinking about a particular thing when "doing" history and therefore don't think about some of the other aspects of the topic.

I'm going to echo what /u/leviathan said about staying grounded among the numbers. For me, I don't really have to work at it, eventually some eunuch will come along and punch me in the gut. I have never really gotten desensitized to it for more than a short period. There have been a couple of times when I've felt myself getting a bit cavalier about the systematic castration of children, that might be the word I'd use. Just, la la la, going along, not really thinking about the human element too much. Right now I'm testing a theory about rates of castration corresponding to Italian economic crises and plagues, oh look they don't match up at all, WELL NEAT, dump the hypothesis. I'm not really engaged in the horror of what these numbers mean when I'm looking at them for that.

But then, in one of the same sources used to build the data for that theory, there's a record of a certain church paying for the castration of two brothers. After the record of payment, one boy goes on to have many other records at the church as a singer, the other boy just... Disappears. No more records for him. What happened to him? Presumably he died, if he'd left the church roster without paying back his castration fee they'd have noted that, that's money running off on you. But how did he die? Why was his death not even recorded in the church payment register, like writing off a bad loan? We can kinda guess the church was too ashamed of what they'd done to record his death. And then it reminds you that all the numbers are little boys and you feel sick. But certain stories kinda get stuck in your heart, and they pop up on you.

In my opinion most historians (at least the ones who decide to pursue their studies in a more professional manner) tend to just naturally become desensitized as they go along. When I was doing my undergrad I slowly became accustomed to the fact that, in my opinion, most injustices of history have never been corrected, and many of the perpetrators have benefited handsomely as a result of said injustices.

I think the people who study history as something more than just a fleeting interest are more likely to come into it with a more cynical outlook on life. I would certainly say, and please understand that this might only be my personal experience, that by studying history I became a more cynical person in general.

Whether I'm studying individual stories or mass quantities of data, I no longer feel emotionally attached to the people or the events in question. I've been called cold blooded and heartless because of the way I usually structure my analysis, but I honestly consider it be more of an impartial way of conducting research. I am not particularly bothered by it, and in fact I think that there is very little to gain and much to lose when you mix in too much emotions.

Not to come off as a sociopath, but I would say that it is preferable for a historian to become desensitized - the sooner, the better. That would allow one to dismiss a lot of data that is not really of any significant value to the matter at hand.

For me personally, I find alot of people have the tendency naturally to get desensitized. I think it is human nature to let go of things, especially if you have no direct correlation to them. Say my Catholic GF cares that the holocaust happened, feels horrible, but isn't affected as wildly as I, a Jew with family that perished, has been affected. I just think its in our nature to let go of these things and realize that it is real and that it happened. It bothers me personally when, say, I find a story of someone getting ripped apart in battle, or summarily executed by Nazis, and the story tears me up. Then you know a day or so later, it is an afterthought. It bothers me that they lived and died and I thought about them for 10-15 minutes tops and they dissipated out of my brain for other information. That may be more of a personal connection to how I feel about leaving a mark on history myself. BUT anyway yeah there ya go

I'm just a history student but I find that when studying I become desensitized pretty quickly but that helps me focus on my work more objectively and can actually make it much more interesting. Rather than just talking about how something was "good" or "bad" I can really focus on what made a particular person, event, or time period work and examine the factors at play in what I might be studying. For example when studying the Atlantic slave trade I did read through some personal accounts that had horrifying details of a slaves life, but after reading a few I was able to string together various commonalities and start to better understand what being a slave meant at the time and different factors that might lead someone to participate in the slave trade as opposed to say, working with their family or hiring workers.