The rational thing for Maria to do would be to end her engagement, allow herself time for her emotional wounds to heal, then hope to find someone else who she can love and who will treat her better than Henry did. But she isn't thinking rationally, she is swamped by the misery of loss and hurt feelings. Feeling as she does, the idea that she would in time meet someone else just wouldn't make sense to her - she is in love with Henry, so marriage to a hypothetical 'someone else' would appear to her to be just as loveless a prospect as marriage to Rushworth.

I think her decision to stick to her engagement is governed by her need to to take care of her wounded feelings lessen the pain she is experiencing as best she can. That pain could be seen as having several different components:

First is the pain of loss of the man she loves. There's not a lot she can do about that. An objective view would be that she needs time to grieve, but from Maria's perspective that pain looks likely to continue forever ("he had destroyed all her happiness").

Then there's the pain associated pain of losing all the exciting feelings associated with being with Henry. When we are in love we experience life as intense and vivid - to Maria the future now looks dreary, dismal and grey, but she can do something to alleviate the situation by distracting herself with new experiences. So I think it is this bleakness as well as the ordinary dullness of life at Mansfield that he wants to escape from when she hopes to "... "find consolation in fortune and consequence, bustle and the world, for a wounded spirit."

Another aspect of Maria's pain , probably the worst of the agony, is that Henry's rejection of her is very undermining to her feelings of self-worth. In effect he has said: "you aren't good enough for me - you were just a game - you were stupid to think I might actually care about worthless little you, you are nothing."
A person with poor self esteem might curl into a sodden heap and think they deserved this sort of judgement, but Maria isn't like that. Her whole sense of self is threatened and it is a matter of emotional survival for her to escape from this pain. Becoming the wealthy Mrs Rushworth with a high social position, a house in town and a glittering social life might in some measure help her to do this.

Finallly, Henry's treatment of Maria is a massive insult, and nobody likes to be insulted. To protect herself from the hurt of this I think she gains strength by feeling angry. She wants to minimise the effects of the insult and doesn't want to give him the satisfaction of knowing how much he has hurt her:

"Henry Crawford had destroyed her happiness, but he should not know that he had done it; he should not destroy her credit, her appearance, her prosperity, too. He should not have to think of her as pining in the retirement of Mansfield for him, rejecting Sotherton and London, independence and splendour, for his sake." She does not want to give him "the triumph of governing her actions, and destroying her prospects."

JA characterises Maria's decision with the terms "pride" and "proud resolve". I have deliberately chosen to use the terms "self esteem" and "self worth" because they are not so negatively loaded, and I find these more neutral terms more helpful in trying to understand Maria's feelings.

As omniscient observers we readers can see that Maria has more pride/self-esteem than is good for her, and we may feel that Maria deserves her pain because she treated Mr Rushworth in much the same way as Henry treated her. But Maria cannot see this, and I find her response to the pain Henry has inflicted as very understandable, and very human, if not entirely rational and wise.