That happens to me too, but not to that extent. I think when you start feeling that you’re going to get stuck, just pause and start the sentence over or just pretend like you’re thinking. This way it doesn’t seem like you’re stuttering, you just come off as choosing your words intentionally. At least that’s what I try and do. It doesn’t work every time, but it definitely helps. This sort of thing happens because of something called a mental block. The only way to get over it is to keep putting yourself out of your comfort zone, not being afraid to stutter, and learning how to speak in a more controlled and calm way. I’m currently undergoing this process.