Why do I constantly make mistakes?

I know how to do a task at work.

I can do the task.

I know what I need to check, steps, but somehow, every time, I get overwhelmed along the way.

It's like my brain just decides to not "let" me see certain things, to push the process along.

Example: I need to prepare an email that says a client owes money.

I check system A for the total of 100 bucks.

I check system B for the total of 100 bucks.

I prepare the email.

I check the email.

I send the email.

I recieve an email back from the client.

WHY DO I OWE a THOUSAND DOLLARS. YOU TOLD ME IT WAS A HUNDRED.

I check system B. It says a variance. The client officially owes 1000.

Where did I read a hundred? ...I didn't. It wasn't there.

I can't trust my own eyes. I don't see the right names, it's like my brain makes up a lie about whats there, I think I have matched totals on a screen when I haven't. I see a name, but it's not right. I stop, staring longer at the information...open to more error because I can't remember what STEP i was even up to.

I go faster, missing information, misreading it, not realising its different.

I stop knowing I have HUNDREDS of things, repedetive tasks, to do....no time to finish, and absolutely no TRUST i can do it properly. My mind throwing procedure out, seeing things that aren't there, not knowing whats next, despite my doing this thousands of times in my life.

I stare at things...and see something that is there, not there, maybe there.

I don't know what is real.

Am I reading this number, or am I not? I recite names, numbers, I check, it's fine.

Until I start to move and get things done, then I get an email back, YOU DIDN'T CONFIRM THIS WITH THE TEAM MEMBER. Oh. I didn't. You're right.

Nobody is perfect, but I can't get this done fast enough, OR accurately enough.

I'm at a loss.

How do other people have jobs and do it all...I'm about to give up.

I can't do this anymore.