Sethe represents the pain and permanent scarring of slavery. Her tortue is both physical and emotional and does not end when she is freed. Although Paul tries and succeeds somewhat in convincing her she still has a life worth living, the lingering effects of her ordeal are nearly impossible to overcome.
Here are some quotes and analysis to support this argument:
'My first-born. All I can remember of her is how she loved the burned bottom of bread. Can you beat that? Eight children and that's all I remember' (Ch.1)
(*Treated and abused like an animal, Sethe cannot even embrace motherhood in a real way. She is too tired and too fearful of losing the children to become close to them.)
"To Sethe, the future was a matter of keeping the past at bay. The 'better life' she believed she and Denver were living was simply not that other one" (Ch 3).
(*It is an out-of-the-frying pan way of life. Slavery was horrible, true, but this life is not better than simply existing.)
"Freeing yourself was one thing; claiming ownership of that freed self was another" (Ch 9).
(*Proves that just because slavery ends, the mind is not free.)