By Esmy Jimenez, Aaron Pressman, and Mara Kardas-Nelson Alexa Arena, a 35-year-old college-educated professional, moved in with her parents to save on rent. Though she works full time, the classic trappings of a middle-class life — the kind she had growing up — now feel out of reach. “I don’t even know what the middle is anymore.” she said. “I’ve been living basically paycheck to paycheck.” Lynn Handly, 61, a state worker living in Brockton, began living alone for the first