Oh no unlucky don't feel bad :(
I have been reading a lot over the past few days - I can definitely recommend "So I'm not Crazy, Stupid or Lazy" which you can buy at the usual places or download as a PDF here: inyer.org/downloads/You_Mean_Im_Not_Crazy.pdf
(Some pages are upside down. Annoying to read on a phone!)
Also my friend, who I saw today, has lent me the ADHD for Adults Workbook. Anyway, a lot of these texts talk about grief when finding out about ADHD, and that we have to process it very similarly almost to grief over death or loss of anything. It's not a loss as such but just a change. But those same seven stages of grief happen - Shock (fairly self explanatory), Denial (it's not that big a deal, it won't affect me, it doesn't matter at all, maybe I don't really have it and am a big fraud), Anger (at all of the past things which you have lost out on due to ADHD, at people not understanding or not believing, why me, why did I have to have this, why didn't I know about it earlier, etc), Bargaining (If I can just find the right medication I will be fixed, if I can just do the therapy I will be fixed) Depression (Realising this is something which is going to be around forever and can only be worked with but not fully "cured", hopelessness, fear of the future) and finally Acceptance (and beginning to manage it effectively, hopefully!)
Not necessarily in that order. TBH, I feel as though I have been through all of those stages at various times, even before giving it a "name" and am now at the last one, but it's possible I'm still actually in the Bargaining stage or it might start all over again given a real diagnosis. Gah. I hope I don't get depressed again. I suppose it's possible, though. I suspect that the grief model isn't relevant only for people who have ADHD themselves but also parents of a child with ADHD or any other additional need.
Another thing we talked about was about telling people. She spoke of a need to go out and shout it to the world, as though to say "Hey! It wasn't my fault all long! Look!" and even met up with old teachers, etc, but found that a lot of people either don't care, don't believe it exists or think it's a bit of an excuse or want to downplay it and see it as a negative. For your mother, there might be a generation issue as well, where a "label" might have carried stigma. Today it's not so much the case, and is just a useful shorthand to allow access to support. Remember, she won't have to tell anybody she doesn't want to, and it might not be necessary to mention it to most people at all. (My friend's mum immediately launched into "Oh don't be silly, of course you don't have that" as though it was something like an imaginary kind of cancer and she wanted to make it go away just by saying so. She did later come to terms with it and accept it, so give her time.)