When Marti Milan, of Claremont, finishes her weekend shifts working as a nursing assistant at Dartmouth-Hitchcock Medical Center, she’s exhausted. She’s been in the job for two years, but these days there are constantly new policies to learn and an ever-present worry about being exposed to coronavirus. All of that makes it difficult for Milan, 21, to shift back into student mode come Monday and do her coursework from Colby-Sawyer College, where she studies nursing.