The book is called 'Cutting it out' by Carolyn Smith, it is on Amazon and I read it as a e-book. It does talk about self harm, and abuse, so trigger warning.

It is such a quirky and deeply accurate description of the way she thought about of her therapist, and what was going through her mind. Would really recommend, still one of my favourite books to date.

It's not very long too. I read the first few chapters, and just couldn't put it down, and finish it all in one go.