Ah OP, this is not your fault at all. In fact, I'd say there is nothing here to blame anything on. My DS has ADHD. It makes school difficult for him, but these kids are who they are regardless of anything we parents have done.
The real issue is that schools and society in general are the dis-abling factor by expecting all children to function and behave in the same way. Focus your anger/frustration on that rather than on yourself.
I've found it helps to look at the positives that come with ADHD. What personality traits/behaviours does your son have BECAUSE of ADHD? For example, my DS is far more interested in a whole range of topics than a lot of kids his age. He never gets bored. If we're outside in the woods/park, he never asks to leave, he can amuse himself with mud/sticks/water/trees/etc for far longer than his friends. And not just outdoors...everything is interesting! Conversely, if he has to stop doing something, he's able to switch his focus quickly onto something else.
So whilst it's tough at times, and my DS struggles at school too, in certain situations my son thrives where plenty of NT kids wouldn't. But
I do think much can be achieved by changing your own thinking around it. There are lots of YouTube videos, Ted talks, books that are positive about ADHD. Immerse yourself in those and be kinder to yourself.