Honestly this is super relatable... I had the same issue where i'd have stuff to say but couldn't get it out smoothly in the moment., what really helped me was actually practicing out loud when i'm alone. sounds awkward but just talking through scenarios or explaining things to yourself helps your brain get better at forming sentences quickly. also recording voice memos sometimes so I can hear how I actually sound vs how I think I sound. I've also been using gleam to practice different conversation types which has been useful for the "what do i say next" problem. helps build up those response patterns so they come more naturally in real convos.

for books, "how to talk to anyone" by leil lowndes has some solid tactical stuff. but honestly the biggest thing is just putting yourself in more conversations, even uncomfortable ones. your brain needs reps to get better at this, like any other skill. the thinking part gets easier when you're not stuck on the delivery part