Time to Let Go (A Bantam starfire book) - Tapa blanda

The doctors assure Erin Bennett and her parents that they can't find a physical cause for her headaches. But the throbbing, violent pain is so intense that Erin knows she needs help. Even landing the lead in the senior musical opposite David Devlin, the good-looking, popular guy everyone else is wild about, doesn't give Erin much pleasure. In fact, she finds David so annoying that her headaches are getting worse.
Erin knows that the headaches started just after the death of her younger sister, Amy, one year ago. She though her grieving was over, but somehow the headaches must be connected to Amy. The therapist Erin starts seeing begins to help her deal with her pain, but what is it about David that triggers Erin's violent reaction?