A BBC article explains the concept of Sisu as follows.
‘Sisu’ in Finnish means strength, perseverance in a task that for some may seem crazy to undertake, almost hopeless.
It has no direct translation in any language (perseverance is incorrect), but the concept is wonderful — I have never encountered this kind of idea in one word. Practically it refers to continue one’s quest in the face of extreme adversities, (and more importantly) even when one is already stretching their physical and mental limits.
Personally, I never knew about this concept but have encountered it in another form. Every day I used to wake up and ask myself: "Are you willing to crawl from Melbourne to Sydney (~900 km)?" The answer has always been a yes. Few people realize that major goals in our heads is the little thing and small steps towards them is the main factor. If you are struggling in this stage of your life, always remember: Where there is life, there is progress. Keep crawling.
What do you do when you have given everything you had? You start from zero again. Do you have the courage to do so?