Katy, I was much older than you when I was diagnosed, but I definitely understand what you mean about not wanting to repeat behaviors in your new family just because it’s what you grew up with. I think that some of that is inevitable, and you might find yourself saying or doing things that you will recognize as coming from your parents (it definitely happened to me, even when I didn’t intend it). But knowing that you want to have something different for yourself and your family is the key, I think. Also, because of the memory difficulties of ADHD, it might be helpful now, before you have children, to write down a vision of what kind of parent you want to be, then revisit that often both to remind yourself and to see if you want to change anything.
It’s great that your partner is understanding and supportive! That will go a long way.
Also, it will be good to keep in mind that motherhood can be a huge stressor for women with ADHD. It is great that you know about your ADHD beforehand. I didn’t, and I always felt that I was on the verge of failing as a mother because I could barely keep it together–even when other people couldn’t tell that’s how I was feeling–especially when I compared myself with the other busy women I knew who had many children (we have only one). Had I known what I was dealing with, I would have been easier on myself and given myself permission to do less, schedule less, say “yes” far less often to other people.