The Breakdown by B.A. Paris

Taking a shortcut home on a isolated road during a rainy evening, Cass, sees a woman in a car that looks like it's broken down. After deciding not to come to her aid, the next morning, Cass discovers that the woman was found dead in her vehicle. Cass's guilt about not calling emergency services begins to eat away at her.  She also starts to get phone calls several times a day where no one appears to be on the line. With memories of her own mother's early onset dementia close in her mind, Cass wonders if that's what happening to her, especially because she seems to be forgetting things. As Cass descends farther into paranoia and absentmindedness, one wonders whether Cass is really ill or is something more sinister going on. The Breakdown is a psychological tale that meanders a bit for most of the book, but the last hundred pages are worth waiting for.