Inadequate training, have made a mistake - meeting with manager

[devoncoastline]

I have been in my role about 18 months. Part of my job involves quality assuring caseworkers' cases. There is a lot of pressure on me to get as many cases randomly checked as possible; I have had two meetings with my manager in the last couple of months where he's told me I haven't been getting enough done and I've ended up in tears both times.

There is a 'sign off' process for caseworkers which means when they first start their job they have to have 100% of their cases quality assured before they can be sent to the customer. Once they get sufficient consecutive cases in a row correct, they are 'signed off' to work cases without 100% oversight, just the random quality assurance of a sample of their cases.
I was subject to the same sign off process and then was expected to start quality assurance. The caseworkers whose work I'm checking have worked far more cases than I ever have and so are much more experienced than I am.

I have made the wrong decision on a case I quality assured and I have a meeting with my manager to discuss this as a performance concern. AIBU to think this isn't fair? I have very little support when doing quality assurance as my manager doesn't know the workings of cases so I can't go to him for a second opinion/advice. The only other person doing the quality assurance in my team is someone who has a similar level of experience to me and so is in the same boat.
I feel really anxious about this meeting as my manager is not very understanding. What should I say?

Have you previously spoke with your manager about the fact you don’t feel adequately trained/competent to be doing this part of your role?

Are you doing this at a higher pay grade than the case workers you are checking? I think you need an experienced mentor to work with - is there someone in another team who could support you?

Why do you think they have appointed you as a checker in preference to an experienced case-worker - was the idea that you would provide a more impartial perspective?

Presumably you have some kind of competency guidance to work through but some decisions will have an element of subjectivity - are there examples of previous quality assessments you could spend time reading through to get a feel for it?

Could you ask your manager about setting up cross-team calibration sessions to make sure you are all on the same page, and where you could discuss any tricky cases?

Essentially you need to be asking your manager, what can I do to avoid making this type of mistake again; and your manager needs to be asking you how they can support you in this.