Please note that I think Susan did go to Aslan's Country in the end.
I own nothing from Narnia.
When Susan entered Narnia the first time she was astonished. Logically it was impossible, yet there she was, fighting a war, no less.
When she became Queen Susan the Gentle she vowed to protect her people with everything she had. Over the years she forgot England. Narnia was her home, her life, everything. It held everything dear to her.
When she went back to England it felt like her heart broke in two. Everything she'd known for the last fifteen years, gone in a heartbeat. Yet there was the hope that they would get there again.
When she arrived in Narnia the second time, she felt like she was finally home. Back where she belonged.
When she discovered all her friends were gone, it almost felt like they left the wardrobe a second time, yet there were the new friends she'd made. Caspian, Trufflehunter, Trumpkin. And there was Aslan. Always Aslan was there for her.
But when she was told she could never go back, her heart broke. She was being kicked out of her home, the only place she'd ever really belonged.
Back in England her siblings talked constantly about Narnia. For a while Susan would join them. After a time she left the room when Narnia came up.
For it hurt too much to remember. She wept when she was alone. She wept for her country, for her friends. She told her siblings she'd forgotten. But she could never forget, not truly.
So she began to try to forget, for a time anyway, by going to parties and dances. It helped ease the heartache for a small season, but when Lucy would look at her with tears in her eyes the pain would come back even stronger than before.
When her siblings died, it felt like she'd left Narnia again, only this time it was worse. Now there was no Lucy to confide in. No Edmund to ask for advice. No Peter to lean on. No one to remind her of what once was.
She was alone. Terribly and fully alone. Even Aslan had deserted her. Nothing she'd loved was left.
Everything was no more. Everything was gone.