This idea of being permanently happy can only be applied to the present moment and things you do in order to achieve your goals. Because happiness is all about intentional actions you take here and now, not about what you think or feel. Happiness is not a feeling but a state of being you consciously enter through the actions you decide to take.
So, what is the best approach on challenging yourself and finding a motivation? Well, in my personal opinion first thing to do is to establish your life priorities and set a goal structure but in a realistic way. What does it mean? It means that we should find our mind-body-spirit balance in order to work hard and rest hard (you know what I mean).