Am I married to my paternal cousin who was breastfed by my maternal grandmother?

Your cousin, who was suckled by your grandmother, is considered her daughter through suckling (rada'a), and thus she is a mahram to you. If the suckling was from your paternal grandmother, she is your paternal aunt through rada'a. If it was from your maternal grandmother, she is your maternal aunt through rada'a. What is unlawful by lineage is also unlawful by rada'a, provided that there were five satisfying sucklings according to the preponderant opinion. If this is confirmed, then it is not permissible for you to remain with her. It is advisable to refer the matter to a Sharia court to examine the details of the issue.