Stamina

Building stamina. It's amazing how doing something new wears you out. 

Doing something new makes your brain work more. You're making new connections, memorising new tasks. The sheer amount of concentration uses up energy. But as you settle into new routines, the pressure eases a bit, and you build some stamina in that new skill/job or whatever. 

Putting in the effort in the beginning is worth it, and although you might feel like you'll never 'get it', it becomes easier as you stick at it.