I have the same thought blaming myself; my daughter was diagnosed with ADHD when 12 and ODD 13... and now we reached in a big impasse (see slapping when I will be able to write about.. still deleting, still writing) she is ending grade 9 and ...
She was passing everything with good marks but I let everything to check and fill up her holes in knowledge playing games or when she asked .. I answered and answered for hours and days when she was interested about something to know. Her teachers always comment that she is capable of much more if she tried. I would say if she wants or ...maybe if she would have taken the medication.
After trying a few pills she read and asked about ADHD and didn't want to take any pill. She even reached to say that she is not having any ADHD or ODD and ask for another opinion, after one of the best specialist (in ADHD, etc) diagnosed her. It has become so hard for me ... and the results in school are going down as I stopped checking something else than that she is in school and in time and she is not having the same attitude as at home, there.

I wished she would have taken some medicine if that would have helped. But she didn't tried for at least a month.

So please stop blaming yourself!