How do I stop freezing when a native speaker switches to fast-paced colloquial Persian?

I’m a false beginner; I can read the Perso-Arabic script pretty well and I know my verb endings, but the moment I’m in a real-world scenario, my brain goes blank. I feel like I'm stuck in a loop of reading without hearing. I’ve started using Chickytutor.com to simulate real conversations, but I’m still panicking when I can’t mentally map the written words to the lightning-fast spoken versions. How do I get past this?