I remember a sitcom from some 20 years ago that I liked watching until one of the main characters would launch into a long monologue on some topic that, obviously, was near and dear to the producers' hearts.
It ruined the program for me.
If several characters had talked about the issue as part of the plot, and in character, it would have been okay. But this was a lecture. A sermon. A finger-wagging scolding to all who dared to disagree with the producers' point of view. (I don't blame the writer. A TV or movie writer does what he or she is told or has the rewriting done by others higher on the food chain.)
That kind of heavy-handedness can hobble a novel, too.
If you want to write lectures or essays or op-ed pieces or sermons, do it.
But don't drop them into the middle of your novel, or worse yet string several together and call that a "novel."
Tell. A. Story. And . . .
Just keep writing.