My favourite book is the diary of Anne Frank. I've always kept a diary since I was 15 mainly but have some entries from age 12/13 then I stopped and started again when I was 23. I find writing does actually help me, but I don't have the words when I'm going through really difficult moments. It's weird looking back and spotting things I didn't see before. I struggle with my mental health- I personally would feel too exposed to let someone else read it.
I prefer real life stories though I go through pretty much anything I just finished a dutiful boy by Mohsin Zaidi. I have also read the book by Princess Diana, Down and out in London and Paris by Orwell, The wrong pair of Knickers by Byrony Gordon, Man's search for meaning by Dr Frankl, The choice by Dr Egar. I let him go by Denise Fergus, My sister Milly etc.
I really would encourage you to write yours. You don't know what your story could offer someone else. Some of the above have moved and changed me in more ways then others but I feel like I've gained something by looking through someone else's eyes.