Part of ADHD. You forget things, beneficial or not. I usually realize when I'm on the way to work, then do the math of turning around for it, and how late I'll be if I do.
One of the downsides of adhd is, if left untreated, it can lead to substance abuse. One of the upsides is it's easier to "forget" that you're addicted to something. I once forget to drink caffeine for a few weeks. Just slipped my mind. I still had the withdrawal symptoms. Headaches, low energy etc., but my brain would not connect the withdrawal to the substance, because from my perspective caffeine did not exist. It was a weird experience.
I've done that a few times. I also quit smoking that way when I ran out of cigs, didn't buy more right away, then forgot to ever buy more. I started back up months later, switched to vaping, then forgot about that too when I moved the vape from it's designated spot to an area of the house I don't frequent.
It's so much easier to be distracted and completely lose track of time with ADHD. Your mind doesn't necessarily think about things that are "boring" like taking medicine. Or even eating. So you forget to do it. I literally have reminders on my phone to make sure I eat because I have gone days without eating until someone noticed me looking all fucked up.
I'll just add that routine is in itself a major challenge - for me, I don't have routines as much as I have laying things out in a way that reminds me to do things regularly. For my meds, I just take it once in the morning, but the one routine I try my best to maintain is flipping the pill bottle upside down. If it's upside down, there's a high chance I either took it, or forgot to flip it before bed, but it's a visual reminder so that I don't need to actively remember to take them on routine, but if I see the pill bottle in a state, I know what action to take.
That's probably one of the hardest things I've seen family members try to understand. I'm not trying to imply anything about you, this is just a related example, but I've had family members see my ADHD family members as just being lazy or intentionally ignoring things, or thinking they're just selfish or whatever. The problem is, even if it's beneficial, a part of ADHD is not having control over where your memory and focus is being put. You may want something, but that doesn't mean you'll sustain attention or effort to achieve it, and conversely you may place it in places you really don't care about to a very consuming degree...
You can't take it past a certain time or you won't sleep. It's not like antidepressants where you can take it at any time. If it's 11am and I remember why I'm so spacey, I gotta wait until tomorrow.
Adhd makes you more likely to struggle with daily routine. I have failsafes to make sure I take my meds nowadays, but I used to struggle a lot with it.
Autistic person jumping in - we too may have issues remembering to do beneficial things like take meds, eat and drink, and even use the bathroom (thankfully usually the body gives a stern reminder before it's too late with that one lol).
Routine is important to many autistics, but personally to me, it isn't really time dependant, so I have the same breakfast every day but it might not be at the same time every day (don't sleep well, sleep patter varies widely), so taking pills at the same time each day is asking me to use a system I don't usually (go by body signals as best I can tell and order of actions, not by time). I hope that makes some sort of sense?
Poor memory is one of the symptoms of ADHD. We get easily distracted and often have time blindness, meaning it's hard for us to tell how much time has passed and estimate what time it is. I have to take my meds 3 times daily roughly 4 hours apart. If I don't set alarms, I'm screwed.
One of my favorite sayings is people with ADHD have two time settings, "Now" and "Later." When you come to us with a request, we think it must be done NOW unless you explain otherwise. Then, it gets puts into the "Later" pile, which ranges anywhere from 5 minutes to 5 years.