In my experience, grammar, as in explanations and tables, is something you need to review over and over, without worrying about what you understand or remember. The key is to start noticing examples of how the language works when you read and listen. Gradually the brain starts to put labels on things, and create patterns for itself. As long as you are just reading and listening, you will not make mistakes, although at times the meaning will not be clear. The meaning will become clearer over time. To become more accurate in using the language, you need to use it , more and more, again noticing the patterns of the language. The more you listen and read, the more you notice the language. The more you speak and write, the more you notice your own gaps and problems. You just keep at it, and gradually improve, hopefully enjoying the process. At least that has been my experience.
It was only when I stopped trying to remember the declensions of German that I started speaking more, and with more confidence, and enjoying the language more. Then I read a lot and listened to a lot of interesting interviews involving real people. The complicated nature of German construction becomes one of the attractions of the language, one of the enjoyable aspects of reading and listening to it, even if we stumble when trying to reproduce these patterns in our own speech.