Well, OP- motivation and discipline are two different beasts. You seem to be looking for motivation when what will help you is discipline.
Set up a schedule. Find the main ideas and then try to weave together what you know. You can make an outline for each chapter (and then expand this to the class as you get closer to finals).
What helps me is to set times. Let's say I have four science classes with their associated labs (completely ridiculous to do to yourself, but proof that it can be done).
Each lab probably falls on its own day (or, you've spectacularly screwed yourself and made two double decker days). You can then assign an hour after the lab (or after the lecture) specifically to that class. Nothing but that class- you can read notes, grind homework, etc.
If that hour isn't available to you, you can always do two hour sessions on Saturday to devote to one class at a time, with a built in break. I tend to get work unequally, so I almost never need that much time. So, I spend some time trying to relate what I'm learning to what is going on, and try to jot some thoughts I have, just to make me think about the material without necessarily running vocabulary drills or anything like that. Saturdays and Sundays are my "catch the burning building" days for if there's a massive project or something like that.
If it's retention that is your issue, you may not be catering to your learning style. Go to your university's career assistance/academic support/student affairs branch and one of them should have an assessment for you. If you're doing everything right and still find yourself behind the curve, then I would look into counseling for possible stress that could be sabotaging you. If all that fails, then I would consider bringing it up with a doctor, particularly if you're also exhibiting symptoms for anxiety, depression, or ADHD.
Low grades tend to be caused by either poor time management, poor studying, poor retention of material, or poor wellbeing. Search, destroy, and power on. You're early enough in the process that you can still get a really solid GPA despite this semester.
Good luck, and I hope this helps you!