I can't speak a second language even though I know it

I can't speak a second language even though I know it I was born and raised in NYC by Polish immigrant parents. That means all my life I was surrounded by Polish speakers. I may not know how to read or write in Polish, but I can understand quite a lot just by ear. When a polish person speaks to me, I can usually understand exactly what they're saying. However, when I try to conversate back to the person in Polish, I just completely shut down. I know the words, I know what I wanna say, I know the grammar, but my stutter physically prevents me from spitting out the words. And with Polish being a slavic language, the sounds you make with your tongue are totally different from English which is my theory as to why I stutter so badly. My grandma only speaks polish, and I wanna communicate with her, I love her so much and I know that her days on this earth are very limited at this point. I want to take advantage of every opportunity I get to speak with her while shes still here with me. But I cant. And it makes me so fucking angry. Whenever I try to say anything, I start getting severe blocks. And she thinks its because I don't know Polish that well, even though I do.. And its not like I can tell her that because It'll take a fucking minute for me to even spit out the sentence. But she's a sweetheart she knows my condition and she simply acts like its not even there.