Like all things, practice makes perfect. Having lengthy conversations is no exception. First of all, though, I can already see that you have built up a mental resistance to longer conversations. This on its own will create lots of difficulties. Your attitude going into conversations is crucial to your performance. So, practice -- you're not handling it well and doing "weird" things because your mind is going into fight or flight mode and you can't focus. When you usually speak there (simple exchanges, ordering food as you mention as an example), you don't have to focus on the other person's words and underlying meaning. Neither do you actually have to put any effort into a good constructive response which moves the conversations forward smoothly. These are a lot more variables that your brain needs to handle! Therefore you lose control of your breathing, your heart starts beating (nervousness, fight or flight because "fuck im messing this up i want it to stop"), etc. My concrete advise to you would to start speaking about complicated things (forcing you to think about more than just your speech) more and more. Important here is to start slowly! Don't stress it. Taking a second to breathe will help you tons. Also, confidence!!! It's just a conversation!! The person you're speaking with wont even remember it at the end of the day. ​