Jett died suddenly in January (09) after suffering a seizure during a family holiday in the Bahamas. He was 16.
Preston had agreed to break her silence about his death at a women's conference in October (09), hosted by Arnold Schwarzenegger's wife Maria Shriver, but the actress has backed out.
She explains in a statement, "It is with my sincere apologies that I must pull out from speaking at the conference. I am sorry, but I truly believed that I could do it. Otherwise I never would have said that I could. But I am still deeply in the process of healing, and it's just too soon."
Shriver, who is mourning the recent deaths of her mother Eunice Kennedy, and uncle Senator Edward Kennedy, has released her own statement, sympathising with Preston's anguish: "I know how hard it was for Kelly to make this decision and she has my complete support and understanding."