But if I say, "I will not mention him or speak any more in his name,"
his word is in my heart like a fire, a fire shut up in my bones. I am weary of holding it in; indeed, I cannot. Jeremiah 20:9
Jeremiah was the prophet least likely to succeed. He was an outcast, among his own people. His life was at risk, his ministry mocked and if the reward of good preaching is converts well one can understand how he felt.
What kept him going? He could only describe it as a fire. No, not a fire from which one could escape but a fire down in the pit of his bones that drove him on.