The Midnight Library by Matt Haig

 Nora is tired of her life--and truthfully of life in general. One night she mixes alcohol and pills because she's had enough. But instead of dying she finds herself in The Midnight Library, a mystical place where she gets the chance to live different versions of her life and see what might have been. Her guide is Mrs. Elm, her old school librarian. Mrs. Elm tells her that when she finds the life she wants, she will stay there. So she bounces from life to life,  as a rock star, a scientist who studies glaciers, a teacher and writer, an Olympic swimmer and a variety of other iterations. And each experience teaches her something new about herself. So when she's about to lose the life she thinks she really wants, she realizes her fate is in her own hands. 

It's a thought-provoking read with an optimistic ending. And you like Nora, so you hope things do turn out well for her in the life she's chosen.