I don't write true crime, but I have worked with it for the last almost 25 years. From policeman to working in prisons to dealing with people on probation for many and varied crimes. Had a sex offender caseload for 8 years.
The best advice I can give is to strive for emotional detachment. I have dealt with everything from murderers to pedophiles to kleptomaniacs. The times when I wanted to say or do something not by "the book," I forced myself to take a short mental or physical break. it was a bit difficult to do that in prison, but you can always go to the control room for a bit.
In context of what you describe, I suggest that if you hit a passage that is difficult for you, work on it when you are at your peak. Some people are best early in the morning, some late in the afternoon. Whichever works for you. Do not try to push ahead, if you are at the end of your most efficient time. Do something else and return to that passage at your next session, when you are mentally fresher.
We have several people on here with the same type experience with the criminal world as I. They may make suggestions also. The key thing to remember is this, when you deal with that type of thing on a daily basis, it is stress-inducing, with high burnout potential.
You have to train yourself to realize when you are at a low point (as I think you may have discovered) and take a break either mental or physical from the situation.
Think of it as re-booting your disgust-o-meter. Hope this helps.