I have problems with social anxiety disorder as well and it's hard for me too talking to my therapist but I think I'm getting used to it the more I go there. But in your case I would like to ask you if it's easier for you to express yourself in a letter? I always felt more comfortable like that because I didn't have to look in the other persons eyes. So I would say you should write down everything you can't say and either give it to your therapist or your parents
join the club.