It has been a while since I read MP, so I do not recall whether I had seen this before. In Ch. 21 when Sir Thomas in a well-meaning way talks to Maria and suggests he would be supportive of her should she want to break her engagement with Mr. Rushworth, it is still amazing to me that she (she's 21, correct) so cold-bloodly "thanked him for his great attention, his paternal kindness, but he was quite mistaken in supposing she had the smallest desire of breaking through her engagement, or was sensible of any change of opinion or inclination since her forming it. She had the highest esteem for Mr. Rushworth’s character and disposition, and could not have a doubt of her happiness with him."

We know short-term, she is rebounding from her feelings of being dumped by Henry Crawford: "...she was safe from the possibility of giving Crawford the triumph of governing her actions, and destroying her prospects;" But it really is a mercenery Maria who is not pledging herself (or even THINKING of) Mr. Rushworth, but "She was in a state of mind to be glad that she had secured her fate beyond recall: that she had pledged herself anew to Sotherton;..."

She is also seeking independence:
"Independence was more needful than ever; the want of it at Mansfield more sensibly felt. She was less and less able to endure the restraint which her father imposed. The liberty which his absence had given was now become absolutely necessary. She must escape from him and Mansfield as soon as possible, and find consolation in fortune and consequence, bustle and the world, for a wounded spirit. Her mind was quite determined, and varied not."