Out of curiosity, have you ever been tested for ADHD? ADHD (Attention Deficit Hyperactive Disorder) has three types: Hyperactive Type, Inattentive Type, and Combined Type. Most of the stereotypes surrounding it have to do with children that have hyperactive type. They often blurt things out without thinking, jump out of their seats, can't sit still, are always fidgeting, etc. But not everyone who has ADHD acts like that, and the disorder can change as people grow up, so adults often have different symptoms from children.
I have Inattentive Type ADHD, and I was diagnosed while I was in college. One of the most common symptoms is that ADHD can disrupt a person's executive functioning in several ways. There are eight executive functions that cover abilities like starting a task, organizational skills, and regulating emotions. Most people with ADHD struggle with at least a few of the executive functions, although someone might be excellent at others. You can read more about executive functioning here: http://www.ldonline.org/article/29122/
Personally, I struggle a lot with boredom. For people who have ADHD, boredom can be almost painful, and a boring task (like reviewing notes from class) might be nearly impossible.
I don't know if you have ADHD, but it sounds like it might be an option. Do some research into "adult ADHD symptoms" and see if you think it fits enough to get tested. It would explain why you can't seem to get yourself to do things even though you are motivated to.
If you do have ADHD, there are several things you can do. First, I really recommend finding a professional therapist to talk things over with. They can help you develop strategies and coping mechanisms and decide what your next steps forward should be.
Second, try taking classes that require less studying and more doing interesting things. Are you able to complete projects that are assigned? If you are building something with your hands? Can you write a paper? Can you give a presentation? Try to take classes that have more of those things and less dull studying for tests.
Third, if you do have to study, don't do it alone. Find a friend in the class or invite someone to study with you. Talk about the material, make flashcards and then play study games with them, or have another friend quiz you both and turn it into a competition. Don't expect yourself to stick with it for hours at a time. Set doable goals like 20 or 30 minutes and then feel good about completing that.
Fourth, If you do end up having ADHD, medication can be amazing. It evens out your brain chemistry to more standard levels and gives you motivation where you wouldn't otherwise have it. It's not perfect and not everyone benefits from it, but if you have ADHD, it's definitely worth a try.
Best of luck!