I am thinking of setting it before he meets Winston in the cells of the Ministry of Love and what he is thinking at that point.
Probably impossible since Parson's didn't have an interior life. He had been completely broken by the state and could only think in the channels that made sense to him. What I could imagine him "thinking" about where he could have gone wrong, what he might have done that caused him to "drift" from the way. And he must also have thought about how great a system he lived in where even your children were trained well enough to spot thought crime and had the fortitude to turn in their errant parents.
The role of Parsons is played very well in the 1984 movie production of the book. If you get a chance to see it before you write your monologue, it might help.