Yes. I have ADHD. Sometimes it’s like my brain just refuses to cooperate. This is more common when I have been stressed or haven’t had enough sleep. I’m a software engineer, and so it’s very obvious to me while working. It’s like the code loses all meaning, or more specifically I can’t keep track of all of the different contexts.

ADHD for me feels like I lack the ability to have background processes. It feels like most people have all of these background thought that stick around, like “I need to check on the food in an hour,” or “once I’m home, I need to look up that address,” etc. I don’t, or at least mine is severely faulty.

“Attention Deficit” doesn’t really explain it. It’s more like attention regulation. My attention is pretty binary. When I am into a thing, it can be all-consuming, and then suddenly it’s gone. I have a bunch of projects that I have just abandoned ¾ of the way through.

ADHD can also make me naturally impatient. I get bored easily, and once I’m bored with something it’s a struggle to stick with it. That impatience can also manifest as thinking I know how someone is going to finish their sentence, and if they are going into detail, my natural inclination is to interrupt to get to the point. I’ve had to learn to be careful about not doing that, and to listen actively and when my brain stopped paying attention in the middle of someone’s sentence, I just fess up to it and ask them to repeat that last bit.

It’s not all a detriment though. I’m good at improvising, and I’m pretty damned clever. I’ve built up discipline to mostly slow down and verify my work. When I am able to focus, getting into a flow state is pretty easy, especially when working on something interesting.

My medication helps me get past those rough days. Not all the time. Sometimes, I just can’t work. On those days, I try to stick with light tasks, and make up for it later.

So yeah, I don’t know if any of this sounds familiar to you. If so, maybe you should talk to your psychologist about getting tested for ADHD. If not, I hope that it helps to know that others out there have uncooperative brains too.

I definitely have these experiences. Still can think about other things, but can't even focus on the topic I was fruitfully exploring a moment ago. And yes, not forcing the issue tends to help, can return to the issue some time later.

Yes, I think this is very normal, though I can't say if it's universal or nearly universal, and I assume thresholds vary. I suspect there are people who've never thought hard enough, long enough to experience it, though.

I have never been very good at predicting when this will happen or how long it will last. Some other people seem better able to do that, some worse. 

I've found that for me, having someone else (not a group, just one or two people, I am very much an introvert in this) working on/thinking about the same thing, and talking to them about it in real time, is often helpful, at least if it is something I genuinely care about; I find that helps me regain some mental energy as long as I'm not too far gone.

Yes. Trying to Think Hard about something logical just makes my mind feel like a brick wall slams down. Things that work: 

-Sticking with things that are easy enough I don't actually have to use Real Brain Power. If I'm learning complex things, the underlying level of abstraction has to be absolutely second nature before I put anything on top of it. 

-Pretending To Think, which works good enough if you just want to trick people into believing that you are working hard at thinking

-Tricking my mind into not recognizing it is Thinking by use of humor, play, narrative

Like Ustice I have ADHD, and have a frequent feeling of low-level boredom that I get around by usually having two tracks running in my brain, e.g. watching TV while working / answering emails. 

I’ve never! Not even close.

I've found taking a long bath is quite useful if I want to think about a specific topic in depth without distractions. At least one of my LW posts was prompted by this.