This is not the easiest article to read. Didion is a sophisticated writer and writes generally quite complex sentence structures, uses low frequency lexis and combines a variety of different writing styles, for example autobiographical and biographical writing. I'm afraid there are a number of articles with similar levels of complexity.
Below is a brief structure of the article to help you identify the main points.
Introduction - autobiographical comments about Didion and her daughter
O'Keeffe and the art world
Her fans and contemporaries
The art establishment
Some biographical details - sketchy from childhood to star watercolours