I have this problem too. You just need to practice, honestly. Read aloud for a few minutes a day and record yourself doing so. Practice with people in real life next. You don't have to do it all the time when you start, just say, "For the next 3 responses to this person, I'm going to slow down." It takes getting used to, but recording yourself and seeing that, even though you feel like you're talking too slow, you're actually speaking at a normal pace really helps. As well as testing the waters with others to start getting used to it.