Wow, your study schedules all sound crazy.
I don't care how introverted you are, everyone needs a certain amount of human contact every day. You need to balance your long hours at the library with conversations/meals with your friends and family. Take some time out of each week to go to the beach, water park, cinema, whatever you usually do for fun. You will find yourself recharged and discover that the process of studying becomes much, much easier.
Also try to get a minimum amount of physical activity (and some sunlight) each day. I try to walk 5 times a week, hit the gym 2-3, and run 1-2. You will find it sooo much easier to sit still and concentrate. I really can't stress enough how important it is to spend a minimum amount of time socializing and exercising to maintain sanity and perform at your best academically.
If you're really stressed, plan a day off in the near future (after you've completed your assignments and work) to do something completely unproductive. Load up the ice chest, pick up friends, drive to the nearest body of water, rent a boat, and go out for some half-drunk water skiing. You'll be amazed at how much acting like a normal 18-24 year old, even for just 1 day per month, can lift your spirits and help you cope with the drudgery of daily life.
You'll probably feel guilty about not studying, but rest assured, it will make up for itself when the next day, you can concentrate better and be less frustrated/stressed.
