Ok, for syllabus,
In essays, everyone performs better when they get a take home test. With history (I dont do it, but I'm assuming), the key is to have a really dense amount of information. I guess thats what yr 11 was for, to improve and make sure you can study well and apply all your knowledge in the exam conditions, which alter your ability to think rationally (thats what I find anyway, I always become frenzied

). Obviously I cant really help with what you're doing wrong, but if you haven't spoken to your teacher about what you're doing wrong, you should. Just seriously, do lots of practise essays and hand them in for marking before assessments.
In essays you should spend 10% of your time (about 4 mins) to plans your response to make sure you dont go off in a tangent. This time is very important and yeah, if you havent been doing it this is probably why your not staying focused and getting spot on marks.