Estella does not love Drummle, and does not intend to be a caring wife.
Pip is horrified when he learns that Estella is going to marry Drummle. Pip has professed his love to her, and she has rejected it by telling him that she cannot love. When she says that she will not be a blessing to Drummle, this means both that Drummle does not really love her and that she does not really love him. She has married him to put an end to her pain in having to lead men on, and he has married her as a prize. She has chosen the most despicable man she could find, both out of self-hatred and as a final slap in the face to would-be suitors.