You can't keep being afraid to be wrong.
You don't have to get everything straight away. Just try to be brave. Make mistakes and learn from them.
Stop worrying about your position in class - it's probably a very good class! You're competing against yourself here. Those guys at the top think they're all safe and have no worries but the only mark that counts is the one in June. Trust me, being bottom of the class is a great opportunity. If you can overcome the fear of being bottom and learn to thrive from it, you'll do far better than you would if you were at the top.
Everything's relative. You may be at the bottom of you class, but you're not at the bottom of everyone.