It is easy to think that once you've turned in an essay, your job is finished - but if you were learning the guitar, or to play football, it wouldn't be enough just to pluck a string or kick a ball, you'd want to hear how the note sounded or look where the ball landed. In the same way, you won't learn about writing essays just by writing them - you've got to pay attention to how other people react to what you've written, so you can learn how to design for a reader.
You can do this while you're writing an essay by asking your tutor or your fellow-students to read through your work and give you comments - this will build up your sense of audience. You also need to pay very close attention to the marking feedback your tutor writes on the coversheet, which you can ask them to explain to you. Numerical marks alone won't give you enough information to improve how you go about writing, because a particular mark might reflect how your essay has failed to meet a number of different expectations.