What causes one to studder?

I’ve noticed over the years that I’m a studderer, if I’m lucky I’ll get one sentence straight before I mess up and feel like I have to say the sentence again, correctly. Oh my gosh, I can’t stand it. I’ve always wanted to be a smooth talker, nothing more but no matter how many times I try to speak out a sentence clearly, I lose it. I’m also a slow talker, so it’s definitely not that I talk too fast.