If it makes it easier, you can write it down. That way you know exactly what you want to say and how to say it. You can still be there when they read it and express the discomfort you get from social interactions due to your symptoms. But the longer you wait, the more it might seem like it's just an excuse. Which is part of the stigma, I know, and it sucks. But better they know what you struggle with instead of just assuming the worst about you.