It seems you are mixing two "states of mind".
Dementia means the mind has forgotten a lot, but is still trying to figure out how the sensory input it gets fits together. This does not result in choppy thoughts/sentences, but rather confused and rambling thoughts/sentences, that are searching for logic. Example: "He had finished his coffee, and the mug seemed to stare at him, asking him to put it somewhere it belonged, but not back into the cupboard, it had to go somewhere else, a place where used mugs go, used mugs out of which coffee had already been drunk."
Post Traumatic Stress Disorder (PTSD) means that the mind can suddenly be brought back into a the high state of stress it was in when the trauma occurred. This might be depicted using short choppy sentences if (and only if) the trauma was one in which the mind worked in short, choppy sentences. Note that this might for instance not be the case if the trauma was that a person was locked in a basement for two years, although such an event might cause PTSD. Example: "He crossed the streets without fear for the first time since the accident, and almost felt pride when a car hooted. Pain surged through his limbs. The windshield smashed into his face. Scrambling for the sidewalk, he screamed nine one one, nine one one, finally being able to scream the numbers that before he could only whisper with his last breaths before blacking out."
I suggest you read up on both dementia and PTSD on (for example) wikipedia and decide which of the two you want to go for, and take it from there.