I keep gettig good marks for interpretation on my papers, but not analysis. What's the difference between them?

You can think of interpretation as what a text is saying, and analysis as how the text is saying what it's saying. If you're getting good marks for interpretation you're probably seeing what the texts is saying in terms of themes or implicit arguments. By focusing on close reading and the use of literary devices such as metaphor, metonym and imagery, how the text is saying what it's saying, and linking this to your interpretation you can achieve better marks on your papers. If an author is employing a literary device, they are usually doing so for a reason. Analysis involves looking at literary devices, mood and diction to determine how the author is communicating with the reader.
Begine your paragraphs with a thesis statement, this should be your interpetation. Start out by stating what the meaning of the text is and follow this up with evidence from the text. The evidence should be based on close reading and the context of the text. This is your analysis. End your paragraph with a sentence which links the evidence you've presented to the meaning of the text.