Not sure this is an ADHD thing but what will happen to me is I’ll be in a flow state, working on something, and feeling that my attention is about to run out while struggling to solve a particular problem. It’s incredibly stressful and annoying to know I have like 10 minutes tops or I’ll be stuck on this problem for a week (because God forbid I break it down into small enough chunks where it’ll be easy to pick back up tomorrow).
I get so hyperfocused that absolutely nothing else is in my brain. After a certain amount of time, if I have to get up and shift focus on something else, my brain is done for. The mental exhaustion hits me. Occasionally I feel the onset. I can definitely relate.