Maybe I can shed some insight on this. I do not have a significant other but my whole family has ADHD and I only found this out (plus the fact that I have it) last month.

I totally get what you are going through, I would always yell at my dad for never taking me to baseball practice on time. I still get furious at my mom for not remembering that I have plans and buying movie tickets for us to go to the movie together.

Now that I realize that I actually have a lot of these problems as well (to not as significant of a degree apparently) I can't help but feel a lot more sympathetic. I always knew my parents loved me but I could never figure out why they always seemed to get so distracted that they couldn't function like all my friends' parents could. Even knowing they meant well, it was an absolutely infuriating environment to be raised in a lot of the time.

One thing though that I have learned from the whole deal is you have to love people at face value. Yeah your SO has these problems and honestly they probably aren't going to get much better. Those symptoms are textbook ADHD, my guess might be that he needs a change in doctor/medication but I doubt his issues are going to vanish overnight. You'll have to accept these problems as an unfortunate side effect of having your SO in your life. If you are unable to do so, than I suggest you reconsider your relationship with him, even if he technically isn't at fault.

As for me, I still love my parents even though they quite frankly piss me off a lot with their disorganization. I decided that the good here outweighs the negative, that's the only way to decide this I think.