He accused prison officers of trying to drive him to suicide in conditions he describes as "a mini-Abu Ghraib".
"If it wasn't for the fact that I am an exceptionally patient person, I would most likely have lost my mind in pure frustration," Breivik said.
The 33-year-old right-wing fanatic killed 77 people last year in Norway's worst peacetime massacre. He detonated a car bomb in Oslo, killing eight, and then drove to the island of Utoya, where he shot dead 69.
Ellen Bjercke, spokeswoman at Ila Prison where Breivik is being held, said he had recently been allowed a normal pen instead of a rubber safety pen.
The self-confessed killer described prison practices as "degrading", including being watched when swallowing his vitamin pills, not being allowed a mop to clean his cell and being subjected to daily strip searches.
Keeping up his personal hygiene is also a challenge.
"Use of a toothbrush and electric shaver is always under supervision. One is therefore under mental pressure to finish quickly ... This limits brushing to once a day and shaving to once a week," he wrote.
"Therefore there is the likelihood that Norway's own 'mini-Abu Ghraib' ... is being kept a secret and that Norway's human rights ambassadors' work to spread the 'world's most humane principles' are avoiding being embarrassed."