Always grateful that I did not join HP, with money in mind, I do not write per "requirements," by any one, at any time. I'm afraid I'm unable to perform on demand, for my writing is only mine to give, to whom I wish, for the reasons, in my heart.
Humans get tired, weary, worn and sometimes listless. Writers do not tire of writing, any more than we tire of living and loving. But rest is imperative and a break from our muse, must be taken, now and then.
Stalking and haunting the writer within, can become a struggle and we dare not feel a need to struggle for what we love most. This will bring on the discouragement you mention....that may be mistaken for exhaustion of writing.....Just step back and breathe a while, give yourself time to refresh. Do not mistake a writer's block for fatigue.....although they may feel alike, the difference is enormous....
Close your eyes and free yourself from all dreary thoughts...just long enough to let your fire rage again. Best of luck and success to you.