First of all, do not despair. Despair is a sin; you must have faith in God through whom all things are possible. There are a number of ways you can get help with this. In order so that we can provide you with the most useful information, if you could provide us with as much information as you can about the nature of your ministry, what background you had before you began preaching, what your precise role is (for example, are you a pastor, or more specifically a presbyter, or a deacon, or a lay preacher?) and what, if any, training you may have received? Also it would help to know what you are preaching about, for example, if you are using a lectionary, if you are preaching in an expositional manner (that is to say, explaining the meaning of Bible texts) or seeking to instruct your congregation in Christian morality, or help their understanding of Christian doctrine, or some combination thereof?
And likewise, are you writing your sermons in advance and getting flustered delivering them, or are you trying to preach unrehearsed, or off the cuff / on the fly as it were (extemporaneously or spontaneously) and running out of gas, so to speak, that is to say, losing your train of thought or running out of things to say or losing your grasp of “the big picture” and having difficulty as a result?