Yea, maybe your brain is becoming conflicted because you’re focused on words ahead in the sentence AND the start of the sentence, so these two factors are canceling each other out, and thus canceling out your speech. You also may be focused on the end of your sentences because right when your sentence is finished, it completes the idea that you are trying to convey. And it’s possible that you are a perfectionist or lack assertiveness so you are afraid that the overall outcome of your sentence or the reaction you will get from others will be negative. And then you stutter on the first word of the sentence because it’s your brain’s way of preventing you from getting to the point where your idea is conveyed