Probably all of us have this book, hidden in our minds. Doesn't matter if you are an introvert or extrovert, if you are an honest person or not, if you are an empath or a narcissist or something in between - maybe one book has only a few pages and the other one is a full collection but we all have these moments when we didn't speak our thoughts out loud because of different reasons.
These words you wanted to say but you didn't, these emotions you wanted to show but you couldn't. Some of these things are not so important and others you'll think about for the rest of your life. How many pages does your book have?
