I (20M) can't talk with my girlfriend (19F). How can I fix that?

Hello everyone. Me and my gf have been together for about a year and a half now, and I've noticed that for the biggest part communicating on somethings with her is almost impossible. She's brilliant, she's very smart, and she can use language in a great way, on the other hand I've always known that I can't really convey my thoughts in a way that's easy to understand, and whenever I was talking to people, I always had to clarify what I meant multiple times. Her family is also not very tolerant of not having solid, direct communication with someone, and she has always said that if someone doesn't understand something straight away in her family, they consider him stupid. Because of all of these factors, whenever we end up discussing something in which we have different opinions on, and I try to explain my point of view she gets mad because she doesn't understand me and then she doesn't want to talk to me. I feel really awful when it happens, and whenever I try to discuss it she ends up telling me that she feels awkward being with someone who can't talk