Here is the great leader of the Reformation, Martin Luther, encouraging a discouraged preacher. The advice stands for everyone. This is taken from the excellent biography on Luther by Roland Bainton, Here I Stand.
If Peter and Paul were here, they would scold you because you wish right off to be as accomplished as they. Crawling is something, even if one is unable to walk. Do your best. If you cannot preach an hour, then preach half an hour or a quarter of an hour. Do not try to imitate other people. Center on the shortest and simplest points, which are the very heart of the matter, and leave the rest to God. Look solely to his honor and not to applause. Pray that God will give you a mouth and to your audience ears. I can tell you preaching is not a work of man. Although I am old and experienced, I am afraid every time I have to preach. You will most certainly find out three things: first, you will have prepared your sermon as diligently as you know how, and it will slip through your fingers like water; secondly, you may abandon your outline and God will give you grace. You will preach your very best. The audience will be pleased, but you won’t. And thirdly, when you have been unable in advance to pull anything together, you will preach acceptably both to your hearers and to yourself. So pray to God and leave all the rest to him.