Hey, I'm studying English Literature at uni now but at A-level I had exactly the same problem.
I'm assuming that you already do this but if not, practice essay questions and ask your tutor to go over what you've written with you. I found this helped a lot for me as they could give me some good feedback about where I had improved and what else I needed to work on for next time.
Linked to this, my tutor advised me to spend 10-15 mins on each question, writing a plan of what I was going to say before I used the rest of th time to do the question. I found this made my work less waffley.
As well as this, I found that forcing a strict "PEER" structure to each paragraph helped ALOT.
POINT - Make an initial statement. For example, "To an extent it could be argued that ...."
EXPLANATION - After this statement, go on to explain your point.
EVIDENCE - Linking to your explanation, use a quote from the text to back it up.
REITERATE - Basically repeat your initial point. " This shows that ...."
I never managed to get a great grade in my exams compared to coursework but I found that this greatly helped me to improve my exam grades.
Hope this helps