First off, keep writing essays and getting them assessed. It may seem like a grind, especially if your grade only improves by 1 mark or so each time, but trust me it is worth it. There are a few things that you should ground yourself in, these things will hopefully help boost your mark and get you back on track:
First off, Context. This term refers to a LOT of variables - the time period the text was written in, the author's own state at the time, and many other things. For example, E.M. Forster's "A Passage to India." The author stopped writing the book for a number of years before he concluded it, and it shows - the style of writing is different, the themes addressed are different, basically the whole thing undergoes a massive shift which completely changes any straight track of critique. As Literature students, we need to take that into account.
Next, you need to look at the text. This is a very broad statement, but I'll go into more detail. First, you need to consider the overall purpose of the text. Is it a critique of government, is it a call to rebellion, is it a forecast of doom. These are very broad things to think about, but you can pick up on them easily enough if you look at the "big picture" themes of the book. For example, you can tell that George Orwell's 1984 is a critique of the Nationalistic practices and Stalinist Idolisation of Soviet Russia pretty easily by reading the book and then looking at the context in which it was written.
Secondly, now that you have the author's aim in mind, you need to look within the text to find out how they go about achieving that aim. In some cases this is more easily seen than others, that's why you have big, easily-spotted metaphors that you can grab and analyse, while others are a little more intimately woven into the text. This may be part of what you teacher may be saying when they as you to analyse quotes specifically. Don't look at the quote just by itself, think of the quote as a bolt in the machine of the author's contention - how does it convey the argument the text presents?
Third, we need to get down into the grit of things and look at words and phrases. This is the most intimate step of analysing something, and you need to have the overall and intermediary level of analysis fleshed out before you start focusing narrowly on one word. For example, if a character says "I am here to hold this wretched country by force" There's a lot you can work with there, but you need to have the foundational context of that statement built up before you cap if off with some top-level word analysis! That way, you can analyse the phrase within the context of the character's actions and purpose within the text, which itself exists within the context of the author's argument.
TL;DR
Big picture to little picture - context --> contention --> content.
Hope this helps, and good luck with Literature!
And P.S.
Everything Hutchoo said about reading good and bad essays is absolutely true - go through them with a fine tooth comb and break them down into their analytical stages and levels. And keep cranking out material - it never hurts to write more!