Being constantly motivated to face challenges is something that depends on our nature and attitude to react or activate stimuli, internal or external ones. From the work point of view it is exactly the same thing, where for example stimuli (or external pressures) come from your boss or colleagues; just mentioned aspects are those that lead to execution of given tasks. Leaving aside the external influence that I have treated in other posts and as just stated are referred to environment, relationship with the boss and colleagues, I want to focus on the internal drive or motivation. There are many small tricks to boost motivation and match set goals; among these, you can find insights in other posts of this blog (i.e. to do list & being craftsman).
Fundamental thing is having trick that helps you in the long run, feeling on being on trial every day is one key; clearly is not something easy to bear, especially if we think of those stressful days where we go to reduce our defense (it actually seems a sort of level of masochism). Having this feeling of constant insecurity leads to setting up mindset of having to earn something every day that pushes you to do the extra mile. I am not saying that you must be feel inadequate or not skilled, but having pressure of possible lost is what you need to do better. Even in real life, as we hold better things that are at risk of being lost, the mechanism is exactly the same for work.