1) You say you are losing motivation - this will kill your ambitions quicker than anything else. If you really want what knowledge your degree program offers you must shake off the negative feedback of your grades and summon up the grit to do the hard work required. In my own case I got a D in my first semester of calculus, I had an evening of despair and then I got angry - by god I'd show those b*st*rds! I got my grades up the next semester and transferred into one of the most rigorous programs my university had to offer (Engineering Physics). The Dean who admitted me looked at that grade and told me I'd be back in a year to transfer out of the program. What a perfect motivator! A new b*st*rd to make eat crow! I graduated from the program - certainly not the best student they had, but I learned the subject I wanted to learn - oddly enough getting A's in what were generally considered the toughest courses and doing less well in others. I've done pretty well in my chosen field after college and am in graduate school on my employers dime. Nobody has ever given a rats a** about the D on my transcript, or even the one in an upper division course that's pretty directly relevant to my work.
2) It seems like its bad now, but it's not. There are all kinds of alternatives to bring your GPA up. Retake classes over the summer if the grades are truly terrible, if C or above leave it alone - use the fear your feeling now and use it positively as a spur to work smarter next year (not necessarily harder - sheer hours of reading aren't always what you need).
3) Students who did well in high school can often have a surprise when they get to the big U and suddenly have to really work instead of coasting on a good memory or raw talent in easier high school classes. You're not competing against every chucklehead who fate threw into your hometown high school - you're competing against the top X% of people from your state/region/nation - whatever. Use this fact to your advantage - find/form study groups with some of those smart people. Used properly it's one of the most effective study tools around - and beneficial in another important way, it keeps you from being socially isolated and introduces you to people who likely have common interests with you, and ideally can perform some of the "buck up, you'll get through this rough patch!" advice that I'm providing now.