But a book is not like a forum post or a tweet, the reader already makes a more informed choice before they read it. Someone reading your book will have either read your previous or be intrigued by the ideas on the blurb. More likely then not they will be prompted to it on a search engine by a previous choice. They will be aware of the type of content that is inside and they will be able to make a proper decision on whether they feel up to the task of reading it.
Besides, such stress or trauma could be triggered by any such things. I knew a person who met a very unpleasant person in a fair and feels edgy around the smell of candyfloss. They could be triggered by a song, a perfectly innocent expression, an eye colour, a tone of voice. It might not be the feelings of despair you describe but the lyrics of a nursery rhyme you mention that send shivers up a reader's spine. The number of people triggered by what you are warning them about will probably be infinitesimal.
Above all, the point of writing is trigger emotions and to bond previous memory with what you are describing. If a person doesn't want to feel something, don't read (or at least use the blurb to pick something more suited to current mood). That said, I'm reading a book called 'The Anatomy of Melancholy' and it makes me life twice as much as it makes me sad.
People who are going through bad times shouldn't be warned away from sharing and reliving those experiences with a fictional character, if anything, it helps them through it. Instead of trigger warnings, there should be huge stickers saying, 'Come! Share this with me.'
