Am I sinful for talking to my maternal cousin and expressing my admiration for her, especially after she broke off her engagement to my paternal cousin, knowing that my maternal aunt and my mother are angry about her decision?

Your conversation with the non-mahram girl and your telling her of your admiration for her is a clear mistake for which both of you must repent. If your conversation with her was with the intention of ruining the engagement, then you have sinned. However, if the engagement had not taken place, then that was not a disappointment. The girl has the right to annul the engagement whenever she wishes, though it is preferable not to annul it except for a valid reason.