It's called a block. Likely a silent block. Your vocal cords lock (either shut or open) and you cant phonate (produce sound). What's worse, the harder you try to get the word out and not stutter, the harder the block resists. The only thing i've found that works is slowly becoming ok being a person who stutters and feeling that it's ok to stutter. Your stutter is mild anyway, so letting yourself stutter on a few words isn't going to great inhibit your communication.