You are wrongfully directing your anger at her. It’s ok to be angry with the situation, but it’s not a fault of your mom. If I were in her shoes, “I’m just making sure that my nephew/niece knows that they have a cousin that remembers her birthday.” In other words, while PWS may feel anxious when having to speak to someone, the reality is that a simple phone call for a birthday wish is just what it means to be part of society. If you stutter, so what? You are only trying your best - and if someone berates you for your best effort, then they are mistaken. Keep trying your best OP, but don’t blame your mom. She loves you and does tries not to view your stutter as a barrier between your relationship with you.