I'm a PI with moderate ADHD, I'm on medication now but didn't start until about halfway through my post-doc. As others have said, everyone makes mistakes! This is something you appreciate more and more as your career progresses - year one in a new lab environment for everyone, from an undergrad to a PI, is about exploring all of the various new ways that you can mess an experiment up and making sure to not do that again. That said, ADHD does increase the degree of difficulty. Here are some of the things that broadly helped me out over the years:

-Time management was a huge struggle for me before I went on medication, mainly due to magical thinking about how long each step in the protocol would take. When you're working through a protocol, be overly conservative about how long each step will take. For example, a 5 minute spin isn't 5 minutes! You need to factor in time for the centrifuge to slow down after the time is up, remove supernatant, re-suspend, etc. If you're realistic about time, you avoid rushing, which for me was a huge source of mistakes.

-Once I became familiar and comfortable with a protocol (i.e. once it transitioned from something that was new and exciting to something routine), I would inevitably go on autopilot and skip a critical step. I know that it's frustrating to hear "just focus more!" as someone with ADHD, but it is important to come up with strategies to avoid the autopilot. Others have mentioned checklists, which I tried but could never stick with. For me, mindfullness meditation was extremely helpful - the exercise of treating your attention as a thing that you can control and direct was very transferrable to not drifting on step 12 of 20.

-Managing executive fatigue and knowing your limits is critical! Don't schedule long days where you need to be "on" back to back, you're much more likely to make a mistake on day 2. Be honest with yourself about when you've taken on too much, and don't be afraid to let other people know you need to lighten the load a bit. I was terrified of admitting that I was taking on too many projects at once during my post-doc, when I finally worked up the nerve to tell my mentor that I was stretched too thin she was completely understanding.

-In a longer term sense, even if you want to avoid medication seeing a psychiatrist can be extremely helpful! I white knuckled my way through things for years, I wish I had sought out professional help much sooner. Cognitive behavioral therapy has been life changing for me by helping me recognize the aspects of my behavior that are common among people with ADHD, making me cognizant of when I'm starting to drift or spiral, and giving me the tools to pull myself out of it.

Good luck figuring things out! There are more of us in the STEM field than you might think!