FWIW my father struggled with CSS for countless years before retiring. The reasons were he never got his head around the DOM or the CSS box model and the fact that it all assumed you were using CSS to style a document (or more specifically, a DOM tree). He would instead want to place this/that here/there, oblivious to the fact that he was building a web page that scrolls, rather than a more familiar VB app screen that does not.

Understanding the DOM, the CSS box model (and more recently, flexbox), and the above-mentioned nuance if you're coming from the app world, is more relevant than ever in our mobile-first time and age. Until you wrap your head around the notion that no, you're not styling an app screen that is fixed-sized, but are rather styling a document, you'll find CSS hard. (And yes, the real WTF is why are we using this type of tech to build UIs instead of native controls, but that's another debate.)

Once you do grok that, you also need to learn to accept some quirkiness and imperfection (it is a lousy language, after all, and browser engines don't render everything the same way), and you'll find that CSS is not that hard.