I have been frustrated by my short memory many, many times in the past. Actually, let me clarify that—I have quite a good memory, and I'm surprisingly good at remembering things that some of the people I know can't. However, when it comes to reading books, I often find myself unable to remember things literally after finishing a reading session.
Obviously, the more time I spent reading, the more this problem began to annoy me. Why the hell can't I remember all the things that I'm reading if the whole point of the activity is to learn new things? Well, it turns out I'm a moron.
The whole point of reading books is to expose yourself to new information and, most importantly, to know where to get that information from. Why, exactly, would you remember everything? Those are some unrealistic expectations that I set for myself. Not only is it dumb to expect such a thing, but even if you had a way to remember everything you read, that wouldn't necessarily be the best thing since a lot of information in books isn't really... useful.
Don't get me wrong—reading is amazing, and it will help you learn a lot. But not every single thing written in a book is going to be important to remember. If you take a book, even a nonfiction one, and you try your best to shorten it to only the important bits, you'll only manage to get a few sentences, maybe a page or two, of useful information. But even then, remembering it all might not work because you still need context, a story, and, well, you get it—it's difficult.
The frustration that we get from not memorizing everything is normal. Considering how much of a fuss everyone's been making about reading, especially in the self-development area, you'd expect that a few books should offer you some information that, if you memorize it, will change your life. But not only is that not true, but memorizing random information that you won't need right now is in itself a difficult task.
You can't expect your brain to just remember random things about a topic that you might not necessarily be extremely passionate about just because you want it to. It will remember information that it either needs very soon, uses on a regular basis, or really likes. Sometimes, context is the reason you'll remember something. Other times, it's the story around it. Sometimes, you just don't remember what you need.
That's the point of books—to store all the information that we can't remember in portable ways that we can access at any time. This way, all you need to remember is where to get the information you need—nothing else.
So, next time you're getting frustrated over the fact that you cannot memorize every single thing from a book you're reading, remember—you're not supposed to. You just need to expose yourself to new information. Some things will stick, some won't. Eventually, you'll start to remember the type of information that you need and where to find it when you need it.