Fíjate que…, or with usted, fíjese que…, means “notice that…” In 2023, the UNAM linguist Josaphat Enrique Guillén Escamilla published a study of mira and fíjate in the educated speech of Mexico City, in the Anuario de Letras. When the markers open a reply, he wrote, they can announce “una justificación, pero también una respuesta despreferida” (a justification, but also an unwelcome answer).
One example from his corpus begins: “Fíjate que oportunidades he tenido” (Notice that I have had chances). The speaker goes on: “únicamente que por cuestiones de trabajo no… lo he podido hacer” (only that, for work reasons, I have not been able to do it).