Defamation occurs when one person tells another a false statement of fact about you that damages your reputation. So if a statement of fact about you is true, that's not defamation. And if the statement is opinion rather than fact, it is also not defamation. True statements and opinion can certainly hurt your reputation, but those statements you can't do much about, at least not with any legal action.
Here the statements made about you were apparently true. You made mistakes in not submitting timely reports, etc. Since those statements are true, they are not defamatory. Your complaint is not one of defamation, but that you made the mistakes because the company did not provide you sufficient training/guidance on what you were to do. That may be true, but that's not something for which you may successfully sue. Some companies suck and just throw new employees into the fray without much training or guidance and expect you to either already know what to do or pick it up on your own. That's not illegal. Just poor management. Maybe getting fired from this job will long term prove a blessing — you may get a new job with a company that doesn't suck and be much happier.