You don't have to discount good things about yourself. In fact, there are few habits that are as harmful. By discounting I mean reframing a positive attribute as a negative, or explaining it away in a fashion that makes it less salient.
I got almost a 4.0 average, getting only one B in my college career? Not that impressive, since it was a state school and a humanities major. I wrote some books that had a positive response from others in my field? "All you know how to do is write your books." I published in a major journal? Oh, some idiots have published there too. Know a lot about Lorca? No, Christopher Maurer knows far more than I do. I know several languages, have extensive knowledge of music history? That's just a mark of "privilege." And other people know more languages. I can do the Saturday New York Times crossword in under half an hour? A useless skill. A good father? Well everyone love their own children. If you are smart, and know it, then of course you are arrogant or full of yourself.
Discounting is easy to do. There isn't a positive thing that can't be turned around against you with almost no effort. It's far worse, even, than the negative self-talk, because it doesn't allow you any answer to the negative radio. If you've been psychologically abused, then you will do it to yourself if you don't take active steps to stop.