Self deception is a recurrent theme in The Chronicles of Narnia. I can think of at least 4 examples.

1) The most obvious example is of course Susan, who by the time of The Last Battle has apparently convinced herself that Narnia was nothing more than childhood fantasies. I say 'apparently' because we do hear this at second hand, primarily from Eustace and it's possible Susan is lying to the others rather than herself. 

I would add that even if true this does not excuse Susan in any way. The usual reason given for Susan pretending to have forgotten is that she finds the contrast between life as a queen in Narnia and an ordinary schoolgirl in England too painful. But this indicates a lack of humility and a hunger for power and adulation. So even if true this interpretation is not very flattering to Susan.

I do think though that Lewis intended that Susan really had forgotten, because self deception is such a recurring theme in Narnia.

2)Another example is Uncle Andrew in The Magicians Nephew who persuades himself that Aslan and the Talking Animals are incapable of speech.

3) A third example is the Dwarfs in the Stable in The Last Battle who convince themselves they are prisoners in a dark dingy stable when in reality they are surrounded by the glories of the New Narnia.

4) Finally we have Pittencream, the sailor who was the last to change his mind about going on from Ramandu's Island to the World's End in Voyage of The Dawn Treader and was left behind. We are told he deserted at the Lone islands on the voyage back and went to Calormen, where he made his living telling wonderful stories about his adventures at the World's End until he finally came to believe them himself.

Can anyone think of any other examples?