I’d like to add my two cents here, basically by picking up on some of the thoughts others have offered. I have ADHD, diagnosed at age 45, was married almost twenty years — a marriage that had a lot of love and a lot of rocky times as well.
Expectations and agreements. If you have no expectations regarding your partner and his ability to be anything other than who he is, then you will never be disappointed and it will be easier to love him just how he is now. If you have expectations, replace them with agreements. (Expectations are toxic and actually cowardly, because if I have expectations that means I am making my happiness contingent on someone else’s behavior. Can I control someone else’s behavior? No. I am responsible for my own happiness.) Or if you have any want or need that requires your partners participation, then go for an agreement. Get his buy-in and his ideas about the agreement and how best to make the agreement work. If he agrees to do something, and then has a hard time living up to that agreement — hint, this will happen and maybe a lot — then that’s an opportunity to figure out what was going on and how to make a new and better agreement. Or to drop the one if it turns out to have been unwise. Get his buy-in and his ideas on how to make the agreement work. Let him use his creativity. Observe if he is ashamed if he fails to keep an agreement or if he knows how to still feel positively about himself even when he makes a mistake. Especially when he makes a mistake. Praise him lavishly when he is doing well and observe if he gives himself credit when he achieves or does something that feeds your relationship in a positive way.
I don’t mean treat him like a child, which he is not. I just mean that it works better to keep things positive and away from shame.
I highly recommend books by Melissa Orlov. A great benefit of this reading is learning how to see ADHD for what it is as opposed to misinterpreting ADHD and instead seeing character flaws and forming negative judgments (which activate shame and withdrawal on his part) and result in a downward spiral.
I wish you both all the happiness in the world … Good luck!