I think I need to work on the whole discipline thing in general
That's a good way to look at it. My wife will be taking the USMLE Step 1 this summer - while one could argue that Step 1's tested topics aren't as disjointed as the MCAT, there's still a ton of material (about a solid two years' worth) to go through and know in precise detail. If you can get a solid routine down and can build up the discipline to study hard, know that it won't go to waste. Once you make it into medical school, there will be plenty of opportunities for you to use the skills you've built up over the MCAT!
Everyone's advice above is quite good. The only thing I'd like to add is that you should do loads of practice problems. Buy the ExamKrackers 1001 questions books, if you don't have them already (not sure if they're included in the complete package). Read your chapters, then the next day (or a few days later) do some practice problems. I've found that things often make plenty of sense when you read them over, but practice problems are the moment of truth - even a concept you thought you understood very well can become problematic, and it's only through those practice problems that you can discover that. I'd argue that practice problems solidify the content in your mind, too, but that may just be me. We're all different, after all
If you're worried about remembering things you read a few weeks ago, there are some things you can do to help. You can vary up your practice problems to include problems from previous concepts. (The EK 1001 questions book suggests doing every third or fourth problem, so that you can go back and still have fresh problems to do. Repeating problems that you've solved before isn't as useful.) Another potentially helpful aid is the ExamKrackers "Audio Osmosis" CDs. Audio Osmosis is basically each lecture from the ExamKrackers books, compacted into 15-30 minute sessions. They run through the material and concepts very quickly, but hearing it if you've already studied it can help to reinforce it. I'll listen to a lecture (or two) while doing laundry, brushing my teeth - any menial task where my mind is free or where I have downtime.
As a disclaimer, I'm not affiliated with ExamKrackers, I just happen to be using their stuff this time around
🙂 If the other companies offer similar products, look into those, too!