For me, excessive overthinking almost always leads to stuttering. I speak better when I don't prepare every word that I'm going to say. If I start talking right away and outpace my brain, I find that my stutter is reduced. It dawned on me that this is how normal people talk. They just start speaking instantly like a bullet out of a gun. Seasoned speakers almost always have a certain rhythm or staccato as well. I don't think it's done to help with fluency, but key pauses here and there serve to keep their audience engaged. I'm not sure if you can avoid negative thinking as much as accepting it. Maybe try treating your negative thoughts like passing clouds in the sky, or the constant hum of the air conditioner. Sure, it's there. Everyone has negative thoughts but it's just pointless noise. They're not real. Try some slow yoga breathing and try conversational affirmations like nodding your head. You'll be a better active listener in no time. Good luck!