In The Master and Margarita, what were Berlioz's drunken reflections on his mediocrity as a reporter as he was driving home one early morning, depressed but honest finally?

I think you may be referring to chapter 6, at the end of which Riukhin drunkenly muses on his poetic life and work. He thinks about his age, thirty-two years old, and realizes that even if he continues doing what he has been, he will never be happy.

And indeed, what then? So then he would go on writing his several poems a year. [. . .] What would those poems bring him?

Furthermore, Riukhin realizes that glory would never come to him anyway, because he is a bad poet, in his own words. He muses that his poems are bad because he doesn’t believe anything he writes.
Later, Riukhin drowns his sadness hunched over a drink, “understanding and recognizing that it was no longer possible to set anything right in his life, that it was only possible to forget.” He both comes to terms with why he hasn’t been successful and is honest with himself for the first time about how it’s his own fault. He is honest with himself about the fact that he is the opposite of Pushkin, he doesn’t believe in his own writing’s ability to make the world better, and he is truly only doing it to maintain access to resources.