i used to have a real problem with this and to some extent still do. a couple of years ago my appearance and my internal turmoil were so incongruent that my parents would be shocked/blindsided when i snapped. i guess i was afraid of not being their perfect daughter and i felt like they wouldn't understand. and i think part of me was right. but after years of therapy, they've become more understanding of my intense emotions and have tolerated me... which helped me trust them more.
i'm still not completely open with them-- and i certainly don't discuss my emotions with friends or my siblings-- but i'm more able to let them know when i'm "not ok" and usually it's via e-mail because i can't stand to see their faces/hear their concern.