Just as a regular library would quench your thirst for knowledge, a Library of Happiness would serve to lift you up when you're feeling down.
It is nothing but:
It need not be tangible. If you're good at remembering stuff, visiting the library could be as simple as reminiscing some of the wonderful events that have had happened in your life.
However, if you normally face trouble recollecting past events, a daily journal could serve the purpose.
Jot down whatever happened to you in your journal at the end of each day. Not once. Not twice a week. Make it a daily habit.
If there is an event that you can look up to, bookmark it or just tear off the page and store it in a separate file - your very own Library of Happiness.
Wondering how to make good use of it?
Whenever you might feel low or demotivated in the future, just take a tour of your Library of Happiness. That's all.
Your memories will get you through the situation.