A Way Back to You by Emily GRay Clawson

What would you do if you were given a second chance?
Annabelle, whom nearly everyone calls Anne, has been stuck in the past for two years. Numbed by grief over her husband's unexpected death and overwhelmed with the responsibility of raising their three young children alone, Anne agrees to let a friend take the kids for the weekend while she tries to get some much-needed rest at her parent's home.
But when Anne wakes up the next morning, she is suddenly sixteen again. And it just happens to be the worst day she spent as a teenager.
High school the second time around brings unforeseen changes and frustrations, but remembering that her future husband, Mitch, has just returned from a mission and is living on the other side of town gives Anne hope. Getting Mitch's attention (for the second time) is more complicated than she could have imagined, but Anne discovers she is stronger than she believed possible-and there just might be a future for her after all.