This reads like: now that I'm old and very wise I finally understand that those who hurt me as a child can't be blamed because they tried their best.
But I don't think absolution makes sense here. The 6 year old isn't at fault for being who he is. It's the responsibility of adults to figure out why a kid isn't thriving. Trying to force a round peg into a square hole year after year after year is not reasonable, and cannot possibly be considered "trying their best". Should teachers continue to bully kids who can't sit still? Is the status quo for ADHD kids acceptable? Of course not. ADHD kids deserve a happy childhood, too.
It's totally fine to make peace with the past and to let go of anger without trying to justify what happened.