It wasn't that she 'forgot', per say. She remembered about Narnia, she just didn't believe that it was real anymore. There are two theories.
First, Susan was always the one with the least faith in Narnia and Aslan. Even at the beginning of Prince Caspian, she is trying to get them all to forget, and just live their lives in London, like normal people. Since young, she was always the motherly type, so she was very mature and practical. Thus, very grounded in the real things around her.
So when it was confirmed that she and Peter would never go back, she didn't actually lose the memories, she just stopped believing that it was real. I mean, even if she spent those 15 years in Narnia as Queen the first time, when she came back, she was back to her original age, etc. So it could very well have seemed like a dream to her. And she didn't hold on to the faith in Narnia, like the others did. She just kept on with her life, and grew into a rather silly woman (like most girls nowadays, actually), losing that part of her childhood. She remembered, she just didn't believe anymore.
The second theory: Susan loved Narnia. She was heartbroken when she was told that she and Peter would never come back. So as a sort of self-defence mechanism, she pushed those memories out of her mind. She convinced herself that it was not real and that it was just a game they played as children, to dull the pain at not being able to return. She told herself that so many times that she actually believed it in the end. [This theory may not be canon, but it sort of fits]