According to a letter to a young fan, Lewis made the decision to keep her out of Heaven because he felt the alternative would be too complicated a story for him to write. He then asked the girl to consider concluding the story herself.
Lewis wrote to one young readerI could not write that Susan was written out of the story not because "Imyself. Not that I have no hope of Susan'sSusan ever getting into Aslan's country" -- that iscountry, Heaven -- "butbut because I have a feeling that the story of her journey would be longer and more like a grown-up novel than I wanted to write."
Lewis admitted fallibility and issued a startling invitation: "But But I may be mistaken. Why not try it yourself?"
Lewis also wrote in his Letters to Children that Susan may one day reach Heaven by a different path, as well as further explaining why she hasn't made it yet.
The books don't tell us what happened to Susan. She is left alive in this world at the end, having by then turned into a rather silly, conceited young woman. But there's plenty of time for her to mend and perhaps she will get to Aslan's country in the end... in her own way.