Preaching is hard. This is the double plus profound conclusion I have come to. It is embarrassing to read back over many of my old sermons. Some I still like. Many I think were just...bad. I'm sure you've had similar experience. The preachers I respect the most are those who struggle with it the most and don't rest on their laurels.
The second double plus profound conclusion I have come to about preaching is that the only way to learn to be a better preacher is to read the sermons of the better preachers. I am honestly shocked that my seminary homiletics classes had almost none of this. I think one professor assigned us to read a couple of sermons. Maybe you had a different experience, but in my classes we spent a lot more time on technique, exegetical studies, Lowry Loops, object lessons, yadayadayada. I learned something or other from all of it, I suppose, but I really wish that I had started my reading of good preachers a lot earlier.
This is one of the main reasons to observe the Historic Lectionary. When I'm called to speak to pastors about worship or liturgy I always bring this up. Even if you are a fan of the post-Vatican II three year series, why not try out the Historic Lectionary for one year? If you do, every week you can read the sermons of Augustine and Chrysostom through the NPNF series (online for free), Luther (online for free), and countless others through the Sunday Sermons of the Great Fathers volumes, and the Gospel volumes of the Ancient Christian Commentary on Scripture. This is a year-long course in homiletics vastly superior to any you have taken so far. I guarantee it. And if you don't love the Historic Lectionary after that year, go back to the 3-year series with what you have learned.