In life, there are many choices, and within a choice, there are many other choices. It is like a tree with many branches within a branch, or a road with many forks within a fork.
Once you have made a choice, do not look back with regret at the choices you hadn't made. Instead, make the best of it, without any expectations. For without expectations, you will have no disappointment. And without any disappointment or regret, you will have no depression.
That is the lesson I took away from The Midnight Library. No expectation = no disappointment or regret = no depression.
In the book, Nora Seed was very depressed, she regretted all the choices she hadn't made as well as the ones she had made. Her life was not what she had expected it to be and she felt she could not do anything right, not even a simple thing like caring for her cat. She wanted to end her life.
In between life and death, she came upon a library. Within that library, the shelves of books went on forever. Each book contained a life that could have been hers had she made that choice. If she found a life that she liked, she could stay in that life. If she didn't like it, she would be transported back to the library to choose another life.
In that suspended state, Nora lived various different lives, trying them out to see if she would like to live one of them. And in doing that, she began to appreciate her original life. It is the book to go to if we ever need to be reminded to be content and happy with our choices and what we have.
