A resignation letter should be short and unemotional. It is not the place to mention your frustrations or disappointments.
Your letter should be two to three sentences at most. It should indicate today's date, then confirm your decision to resign and when your last day of work will be. You might add a single sentence to fill it out.
The person who gets your letter will seldom be the one who wronged you. At least to start with, they have no skin in the game and may even be sympathetic. That evaporates when you start slinging around insults. It makes you feel good to bluster and rage: but it's how the recipient of the letter feels that will matter.
Always remember that your job, writing to a friend, is to entertain. That can mean revelling in the odd pratfall. So, don’t just write about the mundane and pleasant things, try to give them the whole picture and make them feel something.
Your job interview is just days away and you're feeling unprepared. You've reread the job posting about 67 times and you're pretty clear on the role itself-but you can't really think of much else to do to brush up. Don't sweat it.