Oh, man. I see this from so many angles.
As a person (a girl; it's different, not 'worse', just different) who grew up with ADHD, had a severely ADHD older sibling, was raised by a single ADHD parent (diagnosed in late adulthood), was medicated from childhood, and has three ADHD children, I can tell you first off that it's NOT okay for your dad to have treated you like that....but, ironically (wrongly?), I empathize with his frustrations with you. I don't condone it!! I'm so sorry you were treated that way, a child should never be spoken to with those words, and I'm so sorry you were hurt like that.
As a kid, I felt so sad and ashamed of my poor school performance. Some years (medicated) I did very well (star student of the year). Other years (unmedicated), notsomuch. My report cards said things like "careless, not trying, irresponsible, socialized too much". My less cryptic teachers just said, "Talks too much in class!" One teacher even placed one of those cardboard science-fair, Tri-fold boards around my desk to limit my socialization. Still, my grades were pretty decent, though not top scores. The sense of 'failure' was because I wasn't the best, combined with my teacher's words and my mom's disappointment stung a lot. On top of the commentary, I'd get in trouble/get grounded for poor grades at home, ("poor grades" as my mom called them, were "unacceptable", and anything below an A was "inexcusable" because I was "not a B or C student!").
The added bonus was that I was 'chubby'. Not fat, but not svelte either. Side note: I come from a strong line of Viking stock. "Svelte" is not genetically possible with my body. For perspective here, at my wedding I was between a size 4-6 (thin and fit), my wedding dress was an 8 (formal dresses always run small by 1-2sizes), and I still weighed 150lbs. It's like my legs were filled with lead. My 5'1" gran, emaciated from cancer, was still 130lbs when she died. If I were to describe my physique in one word it would be "solid". I only add this to highlight the fact that this was A BIG DEAL with my mom. Growing up, not only were my grades a reflection of her, so was my beauty as a woman (beauty = thin/low weight, if you haven't caught on).
Have you figured out yet that self control issues in ADHD can manifest in food intake? So that was fun!!! Not only was I not smart enough, I was fat. Total fail! I actually battled major food relationship and eating disorders because of it...still do.
As an adult who recognizes and manages her ADHD I still beat myself up for stupid mistakes, my messy, disorganized house, forgetfulness, and of course I still obsess about my diet and weight...partly because any weight loss or gain is verbally recognized by her.
As a PARENT of ADHD kids I find myself SO overwhelmed by their noise, energy, mess, emotionality, forgetfulness, 'carelessness', etc. Its SO HARD to manage THEIR issues along with my own and not feel like it's my fault for their behavior. I feel like I've been a bad parent because I haven't taught them to behave in a store/restaurant/movie theater/friend's house- even though I've told them 1000 times how to behave!!!
I KNOW that when my kids act up in those situations that it's ME who gets the blame from everyone witnessing it. As their mother and teacher of socially acceptable behaviors, I feel like others think that I should be doing a better job. I feel like a failure. I get angry/frustrated/embarrassed by it and take it out on them. I hate that I do that, and I really try to not do that, but it still occurs. It's almost an automatic response to punish them.
So often ADHD is blamed on poor parenting. I don't know your age, but I'm 37. It's only been in the last decade or so that ADHD was widely, publicly, and clinically recognized and accepted as a genetic problem. Notice that nobody blames Autism of poor parenting?!
I almost wonder if your dad was also like you in some ways as a child? Was he harshly criticized/admonished like you and i were? Was he told that his "bad behavior" was a conscious choice and that he was intentionally embarrassing his folks when he acted out, making them look bad as parents, etc.?
Okay, this is a lot longer than I intended. Again, I don't excuse your dad's behavior at all. I struggle to forgive my mom for hers', too. But it helps me to forgive when I recognize where/how she grew up, the mentality of the time, and to remember how ADHD in particular, has been misconstrued among public opinion.
Are you a parent? I don't ask to be condescending, just curious. If you are then you know that parenting is rough. Parenting a 'difficult' kid is exhausting...and sometimes embarrassing. All kids are embarrassing sometimes. ADHD kids are embarrassing A LOT! It's hard to get over that. If you're not a parent, it'll be hard to put yourself in his shoes. Again, his name calling and verbal/emotional abuse is NOT OKAY, but I suspect it comes from a place of not knowing WTF to do with/for you and being terrified you'd wind up as some crazy, nerdowell criminal. But it's obvious that he wasn't taught any proper communication/parenting skills to deal with you. None of us are!
Try to forgive your dad in your heart before talking to him, (don't tell him; that feels condescending to a lot of people). But maybe forgiving him before you approach him with your diagnosis will help you when talking to him.
"Dad, I know I wasn't an easy kid. I want you to know that I wasn't difficult on purpose. I know it was difficult for everyone and I'm sorry if I hurt you. I recently saw X-doctor and was diagnosed with ADHD. It's a condition that I was born with and actually helps explain a lot of the difficulties I/we had when I was a kid.
I want you to know that I love you. I want you to know that I'm not mad or resentful about my childhood and I don't blame you for anything, (note: if you ARE resentful/angry/blaming, then we (you and us at Reddit) need to have a different conversation. But I wanted to share with you my new diagnosis in order to move forward and hopefully build or relationship and offer some clarity as to what was going on when I was little. I recognize how my behavior, while unintended, caused some friction and distance in our relationship. I want a positive and constructive relationship with you, dad. I love you and I want you in my life. (Only say this if it's true).
I'd like to see you and spend some one on one time with you if you're able. Can we plan something?
Love, ...
I apologize for ALLLL the writing. It seems that it came pouring out of me all at once....because ADHD does that.