Just like you used LCSH for your research into the history of anxieties, you will use them to find your selected anxiety in literature and a secondary source about that anxiety and literary work. If you already have your anxiety and literary text selected, put those two items into a search using quotation marks and the word AND. Here's a sample search for agoraphobia and Emily Dickinson:

If you look at the Subjects for the second result, you can see that "agoraphobia in literature" is an LCSH.

Using "xxx in literature" won't work for most anxiety disorders - it will often return only scientific results. It may still be worth trying, especially using the filters to narrow down to literature. Here are some other suggested LCSH search terms for the literary portion of your assignment:
Narrowing your results to only include criticism of creative works will take some trial and error, especially if you don't have a creative work selected. Be patient! And ask a librarian if you get stuck!