I've thought long and hard about a way to describe what having AD/HD is like and here is the best I have so far.
Now I can't truly illustrate what the average normal brain is like, I've never had one.
Now imagine that your brain is two Lite-Brites. The first Lite-Brite is an average size Lite-Brite that many of you had as children. This Lite-Brite is your short term memory. It stays in front of you and carries most of your attention throughout your day. The second Lite-Brite is huge and can hold an amazing number of those little plastic nubby things. This Lite-Brite is your long term memory. Now, the nubs in the first Lite-Brite can only stay in place for a certain amount of time. Either you transfer them to the second Lite-Brite or they are gone forever. If you go back to the nub before it falls out of place you can make it stay in longer and you are more likely to be able to transfer it to the large Lite-Brite. This second Lite-Brite has a very detailed but easy to use filing system. It is rare for nubs to fall out and the large percentage of them stay in for a very long time. So as your day goes by you insert nubs into the Lite-Brite. Each color represents something different. Some nubs are rather boring, such as the what I am wearing nub, or the making breakfast nub, or the driving to work nub. These nubs aren't usually revisited and are mostly not put into the second Lite-Brite. Now some nubs are very important and you place them carefully and make sure to revisit them often so they can be in as long as you need them or can be put on the second Lite-Brite. These nubs can be things you study in school, an important to-do list, or remembering to do your taxes. Now occasionally some of these nubs won't stick and they fall out immediately. Its annoying and disconcerting but not detrimental.
Now the AD/HD brain has most of these things in common but we have some extra special deluxe Lite-Brite features. First of all, the first Lite-Brite is bigger with more holes and can fit many more nubs. Now I go along with my day putting nubs into the first Lite-Brite but instead of putting them in methodically at a steady pace I cram in as many as possible as fast as I can. And I can't stop I just keep cramming them in. The fact that this Lite-Brite is larger means I can put many nubs in that most people wouldn't. This sounds awesome and sometimes it is but its much harder to be positive I put the most important nubs on the second Lite-Brite. The sheer amount of nubs and the frantic pace causes me to not be able to revisit most of my nubs. I have to scan them all every few seconds and try to hold onto the important ones. But many times when I try to scan over them, several of them fly out and bean me in the face, or I keep going back to the same nub over and over and over and over and over even if its not an important one. Or, my favorite, the nubs start changing colors and the board starts to spin. Which nub is which? Another thing that happens is called hyper-focus (a real AD/HD term). This is where I ignore every single nub except one which I stare at for long periods of time. All the other nubs fall out but I don't even notice. Now this makes it much harder to get nubs moved over to the second Lite-Brite. It's also hard to use the second Lite-Brite when the first one has to be watched so carefully.
Now I don't know if that makes much sense but its the best illustration I could think of to come close to what I deal with every day. You can't really blame me for wanting a cookie after that.