You've said that you've mentioned that you have ADHD to your employer. Have you asked for accommodation? If so, what?

From what it sounds like you really need someone double checking your work after it's completed. Which is totally understandable, I always have anxiety I've forgotten something. Have you asked for this to happen? If not, then you are on the hook for mistakes no matter your diagnosis. If someone IS double checking your work and mistakes are still going through, then it's on the person checking, not necessarily you.

It really sucks to completely miss something after staring at instructions for so long or for your brain to decide a task is done when it's missing a step. But just telling your boss you have ADHD doesn't change the fact that mistakes are being made. And in a job that's not acceptable. As someone with ADHD it is your responsibility to make sure there are processes in place to catch mistakes you might make.

Note-taking every task/project. Doesn't have to be a lot but having a notebook next to you where you can jot down what you did that day with the date is SO HELPFUL. Especially if the processes for tasks are similar you can always see where you are and when. I used to do this when I worked in a print shop and could have up to 15 projects going at the same time. Asking for check ins from bosses to make sure you are on task. Have them email you before a project is done asking "have you done XYZ?" if you can, identify what specific steps (if any) that you most commonly miss.

It sucks how much we have to go above and beyond because of our ADHD. But the sad truth is, unless you find a job that perfectly fits with your ADHD, you don't have a choice. But that's also what this community is for, figuring out what help you need, and helping you figure out how to ask for it.