When I hear people say that writing is not fun, or that they do not like to write, I am reminded of the difference between pleasure and joy. Pleasure is an experience that is more related to the moment, to that which provides immediate gratification. Joy, on the other hand, is not an experience of immediate gratification, but is more about an overall experience of satisfaction. Yes, at times writing does not be great pleasure, but it usually brings deep joy when we push through the difficult time.
Try to not give in to frustrations. Allow yourself to feel them, and push forward. Very little of great meaning in life is easy.