My wife was diagnosed with ADD (ADHD-I) at 20 (no points for H at all, literally none).
When all stimuli hit you at the same level it's really tiring. Someone clicking their pen, bird chirping outside, someone coughing in the hallway or the classroom next to you, someone picking his nose in the corner of your eyes etc. She isn't able to filter them at all.
I've come home to the place being flooded because she ran a bath but remembered she needed to do something in the kitchen, and that prompted something else, etc. We've fought because of little things, but those little things combined with having to process every single stimuli the world throws at you is difficult. If you're focusing on work, or just sitting around do you even register the things going on outside? She does. At the same 'volume' as the thing she's trying to focus on.
In high school she missed more classes than she went to. She was deemed lazy, dumb, and uncaring. We have three levels in the Netherlands and she barely completed the lowest one. Mainly because she was home sleeping, or just wandering off with her mind, migraines - you name it.
When she had to write school reports for a specific class the teacher would be baffled by the report. It went from the actual subject to a few pages of an English report, then a chapter of economics. I always imagined it as her having to go to the bathroom, coming back and remembering 'Shit I needed to write that book report for English class'. Because, obviously, this was only started just before or even after the deadline.
You'd think the schoolsystem would raise an alarm right? Nah, she wasn't exactly 'trouble' as in being busy / shouting / name any typical bad behaviour. She was just chaotic and 'dumb'. Real easy conclusion, especially when you have kids getting in fights and smoking joints - at least she was trying to do her homework.
So she missed a lot of opportunities, felt really bad about herself to put it lightly, and the migraines are a bitch too. I don't know if that's how bad it can be when undiagnosed but yeah. When the psychiatrist did the sessions / tried medications they also took an IQ test which indicated that when the world is not throwing stimuli at her (well it still is, but with the help of the meds she can filter it) she's actually really fucking intelligent (remember the lowest level she barely passed, could've easily done the highest level and moved on to university). After years of hard work catching up she did complete a BA and I couldn't be prouder. It's not easy believing in yourself after so many years of people saying you're dumb and lazy.