Yes, as Zurich said, once you're behind it takes an awful lot of work to catch up again. I don't know how popular this would be with you but... how about setting your alarm clock an hour earlier, and reading for an hour to try and catch up on the books at least.
Your brain is probably feeling tired because it knows how much you've got to catch up on, I think the best way to go about this is to try and break everything down into chunks. Manageable chunks, and realistic targets. Once you start achieving these targets and getting back on top of things, I'm sure your motivation will return.
One of the most important things is to get yourself into a reliable sleep pattern, with no excessively late nights, and make sure you relax before you go to bed so that you can actually get a good nights sleep instead of a disturbed one, which is likely to happen if you're worrying about things when you go to sleep.
Be determined. Again, I agree with the working together suggestion - working with other people definitely increases your motivation, so I'd definitely give this a go.
If you have space between lectures, pop to the library or something, just, whenever you feel yourself not really doing anything constructive, go to a different environment and read your books.
Try not to miss any more lectures - even though you're behind, by being there, you'll pick something up at least.
Hope some of that maybe kinda helps? Hope you manage to get into some sort of routine. Good luck x