Antoinette gradually loses her identity and forgets who she is after she marries Rochester. She craves his love and also very much desires peace, but Rochester does not really love her and so cannot give her what she needs.
The marriage was brokered only so that he could have the economic security of her vast fortune. Nobody looks after Antoinette's interests properly, so control of her entire fortune falls into Rochester's hands, leaving her without even an allowance to fall back on. This economic powerlessness contributes to Antoinette losing herself.
Rochester, an insecure man, succumbs to rumors and a poison pen letter to accept that he was tricked into marrying Antoinette, a woman he now believes is destined to become insane like her mother. His affection (never love) for her turns to hatred, because he feels he has been deceived. He sets up secret and unfair little "tests" of her love and faith in him, which she inevitability fails, driving him further from her emotionally. He also increasingly dislikes and feels alienated by her Jamaican home, which is where her soul lies.
Rochester begins to erase her identity by calling her Bertha rather than Antoinette, replacing her exotic Creole name with a stodgy one of Anglo-Saxon origin. Worse, he takes her back to England, and certain she is destined for madness, has her kept locked up on the upper floor of his home. All of this drives Antoinette to madness: she needs love, caring, nature, and freedom, and yet she is trapped and powerless, far from her home and caught in a world she can't understand.