This personality article is an opinion by a fan and should not be considered canon.
The symbolism and significance of Susan's character has been much discussed. In particular, her failure to enter Aslan’s Country at the end of The Last Battle has prompted a great deal of speculation among readers.
Character-wise, Lewis may have intended Susan to represent the good seeds that are "choked by thorns", as in the parable of the sower from the Gospel of Matthew. Polly Plummer claims that Susan's "whole idea is to race on to the silliest time of one's life as quick as she can and then stop there as long as she can." That is, Susan's failure is due to vanity and a false adolescent sense of "maturity". This provides a striking contrast to her sister Lucy, who is a shining example of the Biblical "faith as a little child."
It has been argued that this inclination towards maturity gives Susan a sense of self-reliance, or a “lack of faith”, that prevents her from following Aslan. In this interpretation, Susan may represent those who, in the confusion of their fallen state, find a spiritual call to faith drowned out - not by any malice on their part, but simply by the mundane distractions of everyday life.
It can be argued that in his portrayal of Susan, Lewis is attempting to illustrate the importance of keeping things in focus: By devoting her entire present life to the temporary and superficial (i.e. “nylons and lipstick and invitations”), Susan sacrifices her chance at something eternal. There is no indication that her natural maturing is inherently wrong, but to become overly devoted to the petty and shallow aspects of life is.
Absence from Aslan’s Country
In a letter written to a young fan, Lewis stated that Susan's story was not finished. Perhaps he was saving Susan for something more: He had planned to write a book called Susan of Narnia, to reveal what became of Susan after The Last Battle. Unfortunately, Lewis died before he could start writing.
There remains controversy among fans as to whether Susan's absence in Aslan's Country was permanent. It is possible that once she remembers Narnia as it was and remembers her place in it as Queen, she will be able to go to Aslan's Country when she dies.
It can also be pointed out that the other children enter into the "new" Narnia (representative of the eternal Heaven) because they have died in a train accident, while Susan remains alive on our world, providing no proof that she has been permanently excluded. Aslan's last words at the coronation of the four Pevensies offer the best justification for believing Susan will eventually join the others in Aslan's Country when the time comes: "Once a King or Queen of Narnia, always a King or Queen of Narnia."
Another thing that should be noted was that at the Pevensies’ coronation, was Aslan saying "may your wisdom grace us until the stars rain down from the Heavens". However, since Narnia had already been destroyed and "the stars rain down" before Susan died, this may mean that Susan has missed her chance, but since many of Susan's friends from the old Narnia are in the "new" Narnia, and everything was "the same, but better", with the "new" Narnia having night, the stars are in "the Heavens" there, which could mean that Susan will join the others when she dies.
Also, Susan must have greatly suffered when told that her parents, siblings, cousin and friends had all been killed. Maybe upon learning of this, Susan would begin to see life more clearly, and become like she had been as a Queen of Narnia, foresaking her silliness and embracing life again, and prepared to accept disappointment, finally joining the others in the "new" Narnia when she eventually dies.