So you KNOW the problem, which is good. A lot of people pretend not to even know where to start that's frustrating for both parties.
1. I recommend this tool here.
Anki - friendly, intelligent flashcards
Most people don't have the discipline to use it, but these people tend to be the kinds that don't do well at school anyway. So here you go. Just read the site and download it. It's essentially a flashcard program but it does most of the organizing for you.
If people asked me why I get all A's without trying much, that's my first tip.
The software utilizes something called SRS. Read this site for more information:
Super Memory
The tool pretty much guarantees you'll never have to study for finals ever again if you have the discipline to use it consistently from the beginning of the semester.
2. I have no problem with you reading extra. In fact, it actually brings tears to my face knowing some people out there are go off the beaten path. Most students I encounter ♥♥♥♥♥ about school 5 days of the week and get drunk the other too. You seem to enjoy it too, which is good. I burn myself out doing what I love every day. I typically wake up early the next morning ready to burn myself out again. I don't really have the heart to tell you stop reading extra.
3. I know this is cliche, but I gotta say it. Set clear academic goals. You make a conscious effort to read extra. It's not some uncontrollable habit, you're the one who chooses to read extra. So type out your academic goals and use them as bookmarks for all your textbooks and paste them on every notebook you own. Therefore you get constant exposure to your goals. Every time you're about to read extra or do something detrimental to your grades, read the goals. Ask yourself. "Have I done all the necessary coursework that's due for all my classes before I read extra." Reading extra is not part of the course, it's a hobby. So just prioritize your required academic activities and those that are not required. Set specific times in the week when you all you do is required academic activities. Set some other time preferably a whole day for other activities. Maybe, Friday. Make that your unrequired coursework day, as well as your errand day and housework day. Everything you do that day is not required. Every other day is required coursework.