As in having the inner strength to keep pushing forward with your writing no matter what.
That's what I do. The shove! Lots of frustration, questioning myself as a writer, telling myself to push forward, happy the next moment that the words are effortlessly being written. And when one is written, the great sense of accomplishment. The roller coaster ride.
In order to be a writer, besides physically sitting at the computer or notebook in hand, takes guts. Because truly you are putting yourself out there. I'm sure I'm not the only one who will have slight panic attacks after sending off the query letter, synopsis, and/or manuscript, questioning myself about the quality of my own work. That's where the inner strength comes in. Because even amongst a rejection, you move on. You either try another editor or agent or if there were suggestions, you may revise it.
My point is in order to write, you need that inner strength to get through all the crazy thoughts and times along the way.