What does Linda say is realized by every slave girl around the age ten & the source of hatred and jealousy from their mistress?

That would be the slave owner's desire to sleep with his slaves.

She felt that her marriage vows were desecrated, her dignity insulted; but she had no compassion for the poor victim of her husband's perfidy. She pitied herself as a martyr; but she was incapable of feeling for the condition of shame and misery in which her unfortunate, helpless slave was placed. Yet perhaps she had some touch of feeling for me; for when the conference was ended, she spoke kindly, and promised to protect me. I should have been much comforted by this assurance if I could have had confidence in it; but my experiences in slavery had filled me with distrust. She was not a very refined woman, and had not much control over her passions. I was an object of her jealousy, and, consequently, of her hatred; and I knew I could not expect kindness or confidence from her under the circumstances in which I was placed. I could not blame her. Slaveholders' wives feel as other women would under similar circumstances. The fire of her temper kindled from small-sparks, and now the flame became so intense that the doctor was obliged to give up his intended arrangement.