The breakdown by B. A Paris

Cass Anderson didn’t stop to help the woman in the car, and now she’s dead.
Ever since, silent calls have been plaguing Cass and she’s sure someone is watching her.
Consumed by guilt, she’s also starting to forget things. Whether she took her pills, what her house alarm code is – and if the knife in the kitchen really had blood on it.

https://www.amazon.co.uk/Breakdown-gripping-thriller-bestselling-author-ebook/dp/B01AKV8EKQ/ref=sr_1_1?s=digital-text&ie=UTF8&qid=1488902719&sr=1-1&keywords=the+breakdown

So Mathew asked Cass not to take the sort cut cos it'll be dark and the storm is due. She decides to take it against Mathews wishes and see a car broken down and cos it's dark and the storms brewing she doesn't stop to help. So when next day she hears about the murder of a young woman on the road she went down, she worries what would've happened if she stopped and help would she have been murdered or not. 

But then the identity of the woman is revealed and cass realises she new the woman and had lunch with her week before and is now feeling guilty for not stopping. And to top it of Cass is worried that she is going mental as things are happening that she can't remember planning or doing.

So after Cass nearly overdoses she decides to start taking control of her life and that when she discovers the horrible truth of what really happened to Jane and who was behind it all along.

I was really shocked when I got to the end I never expected it to be who it was always thought it was a random murder so glad I decided to give this book a go and can't wait for the next book of this author to come out.