Maybe it would help to keep the first session / intake rather light? I'm not saying to lie about anything, but you don't have to get really deep into things if you are uncomfortable doing so. For example, if you experienced physical abuse as a child, you could say just that and tell them you don't want to get into details right now. I also sometimes find it a relief to just be direct when I'm worried about something like laughing or smiling inappropriately. I don't have that exact problem, but it seems like it might help to just let the therapist know that you're nervous and it might happen. Then you don't have to keep thinking and worrying about it and what to do if it happens.