I've been struggling for about a year in my English Literature and History A-Level classes as I just can't write a persuasive, flowing, coherent and comprehensive essay. The main problem I think is that my structure is messy and I stray from the topic a lot so the teacher marking my work gets confused. Another issue with my work is that it is rarely persuasive, but I don't really know where I am going wrong or why. I would love some actual advice, so far all I have got from my teachers is 'oh, make it more concise' but I need actual instructions on what to do next, what to change, because I'm a hard working student and I will literally do what ever it takes to get an A this summer.
What we can offer here is rather limited. If you need more help, there are many great tutors available to work with your particular need(s).
The best advice I would give you is to take time before you write an essay. Don't focus on what position you plan to take, but focus on the examples you should use. Take a position that you can defend well. (Also, notice that I said "examples". For persuasive essays, as much as possible use concrete examples, not just reasoning.)
Then, take time to make an outline and pre-write your thesis statement. The best way to organize your essay may not be the the way the examples come to mind. Organize them in a logical way.
When your done writing, take time to proofread.
One more thought ... If you haven't done this yet, re-write one of your essays based on your teacher's comments. Bring it back to the teacher, and you should understand better how the teacher wants you to write your essays.
I hope this helps.