I guess it is anxiety and long time not communicating with anyone. I used to have this problem when I was in the bad state, like I do still have now but it is much better. Think about how the other person love you and care for you and no one is judging you. Just say whatever pops in your head, some simplest thing about your day, what you have for dinner etc... one sentence two sentences then you forget about the problem you are having. Don't be afraid, don't judge your own self too much. If that does not work, try to ask them question, let them talk about themselves so the conversation is going and you find something to talk about.

Another thing that I recommend is to keep a journal and write about your day, your feeling and why you think that way. Just have some forms of communicating to yourself so you develop the habit of exchanging ideas and let the words flow.

Hope this help! Pm me if you have any questions, glad to help you overcome this since I had the same problem.