Hey tamamo,
Certainly not a bad start! You're in 1&2 so the most important thing is to get structure right. I'd suggest making sure you include line/page numbers for every AC that you do, so maybe try and add them into this AC you've done here?
In your introduction, I suggest you stick to the FARMSS principle (Function, Audience, Register, Mode + (Text Type+ Author), Social Purpose & Semantic Field). This enables you to outline the general areas of discussion for your AC, and how the use of lexical and syntactic choices influence them.
Taking a quick look at your AC, you've covered the general concepts of FARMSS, but the Function & Social Purpose needs to be tightened up IMO. For example, when you discuss the purpose being 'to show humanity', I'd say that isn't awfully clear, and would suggest going for a sentence like 'the function of the text is to inform the readers of soldiers humanity in WWI', with the further social purpose of 'providing information about the sacrifices and decisions made by soldiers in war to a younger demographic of readers so that the knowledge is passed down to the younger generations'. From here, I'd say specifying a field (eg. war) referencing some related words or phrases would be helpful to tighten up your AC. I like your final sentence, but I think clarifying that the formal register mirrors the language used when the book was set (eg. use of obsoletes, I haven't read it so I'm just guessing) would add to your AC immensely.
Your main paragraphs follow a similar line, some great evidence and knowledge in there but it just needs a little structure, and making sure you use line/page numbers!!! I'd suggest asking your teacher about any key overarching principles that they think should be used in each paragraph to then proceed using metalanguage to analyse. I like using what I outlined in my introduction, so having a paragraph about big ideas (eg. how choices by author link to register, social purpose etc.), but I also think it's vital to talk about the context, so I'd suggest highlighting that in your piece.
I hope this helps! Feel free to ask anything that you think needs clarification.