This is one of the most challenging things about having a stutter. Needing to think clearly about the topic you're trying to talk about while your brain simultaneously tries to think of alternate words you won't block on. I've found it's most difficult for me when I have to make prepared remarks about a topic. I'm much better in extemporaneous or conversational situations where the points I need to make are being teed up by someone else in bite size chunks. When I have to think about laying them all out in a clear order, my brain goes a little haywire.