Every job has a learning stage, it is natural to have some shortcomings at the beginning.
If there are certain things you can improve upon, then use the letter to teach you those things. After you finish clean up an area, why not ask your boss or supervisor to give it a quick look so they can tell you immediately if you did it correctly or if they expected something differently.
You have mentioned that you quite often get mad at yourself and then do some sort of violent act on yourself. Do these acts help you at all? They are how you react, but if they’re not helping you, then why literally hurt yourself when you may be able to address the situation/problem at hands? Could they be making it worse, because by doing so, you make it harder to think logically about the situation you’re in?
You have been expecting to lose this job for a while. There is some truth to the self-fulfilling prophecy. Could it be that you think you’re going to lose the job anyway that you are making mistakes that would lead you to being fired?
If it’s all you think about, there will be a part of you that believe it’s going to happen and you will make it happen.
It’s scary and it’s hard, and I’m sorry it’s been a rough patch you’ve been in. But you need to learn to regulate your emotions and find ways to manage your anger with yourself. You work in a school (I think?) and therefore there are standards of behaviour you must think about. The kids shouldn’t see you punching yourself or anything like that.