Think of it in reverse. What would a day look like if you really focused on one thing. Presumably you would be worn out if not after 4 hours at least after 8. Now you would have another 4 to go. That isn't sustainable. Doing all sorts of random things that doesn't need to produce a result probably is.
If it's actually impairing your life (as in, you put in 12 hours days because there is no way your brain will allow you to complete your work in 8), you need to consider changes in work environment may be needed to keep you on task.
Several of my friends had success with lists and time tracking (it helps be aware that, whoops, you're working on bug #Y now). Pair programming tends to keep people very focused and it will never go on for 12 hours, so you could pair part of the day, get the core work done, and then feel guilt-free about the self-learning.
Take physical breaks, start drinking coffee, or both. Sometimes lack of focus is a natural consequence of your task being not nearly exciting enough. We all have tasks to do that are terribly boring, so when stuck in one, occasionally give your brain a nugget of stuff feeling new. But don't task switch without physically changing your environment; going outside when you need to goof off will let you internalize that at desk=boring stuff we get paid for and outside=play time. (Like many other smart children you and I probably didn't learn that lesson quite as well as we should have in school.)
All you need is 40 hours a week of solid productive work. that's it. Get that in your head and work from this first principle.
make time for downtime - allow your mind to wonder and be bored and go off tangents sometimes. Counterintuitively, that's part of the magic. do some breathing exercises throughout the day(if you hate meditation like me lol)
Get a gym membership and do a rigorous workout 3/4 days a week.
Become friends with water: seltzer, regular, fruit-infused, tea, it doesn't matter. drink lots of it. It'll have 2 main effects: keeps you hydrated of course(with the many benefits) and it makes you get up every few hours to go on a break.
* Assuming your sleep is fine, have you ever been checked for ADHD? Medication has helped me be less impulsive. I call it my "anti-F-it" medication. Where I used to think "I shouldn't really go off to do this, but f it", I now think "I shouldn't really go off to do this -- so I won't." Not a silver bullet, but a noticeable improvement.
* I've struggled with focus vs. exploration since childhood (I'm 43). It has never become easy, but I've become much better at it through experience and training.
* One dynamic I have is overcommitting time-wise. I know I don't really have a lot of time to do task A, but f-it, I'll do some B, an it should just work out. Oh, I didn't finish my work tasks by EOD Friday, OK I'll just work the weekend. Can I do this in a week? Sure. A week is effectively infinite time to my knee-jerk-reaction task planner.
To combat that, I now put all upcoming deliverables in my calendar. I have a TODO note in Evernote separated into TODAY, and BACKLOG. Anytime I think of a cool project or idea, I put it in BACKLOG. The day's tasks go in TODAY. To avoid over-filling the day with tasks, I actually block out time for each task in the calendar. The idea is not to rigidly stick to the calendar, but to get constant reminders that there isn't infinite time in a day or in a week. Sometimes you really can't make up the lost time later.
* I track my time with Toggl.com. Not everything, but studying and software development projects. I now know that it takes 1hr to do a 2 question thrice-weekly physics assignment. If I ship some personal project, I'd like to have some idea of how much time I actually spent on it. $/hr and all that.
* Try Pomodoro. This got me out of a sticky situation a few years ago, but I stopped using it. Have started using it with my kid, for homework, for good effect. These days I only use it when I notice myself procrastinating.
EDIT> Also, as many others have said don't work 12 hours a day if you can avoid it. It's killing your long-term productivity. I have been burned out for years, and am only now getting out of that.
1. Don't work 12-hour days. Work 8-hour days. Seriously, overwork is BAD (for you, and for your employer).
2. Focus comes from understanding your goals. If you know your goals you'll be more motivated to focus: https://codewithoutrules.com/2017/08/03/stay-focused/
Also, make sure to get enough sleep. This can have a direct impact on your ability to focus. 12 hour days can get in the way of this.
I suggest finding a project, feature or initiative at work that ignites your passion and puts you into flow (as defined by Mihaly Csikszentmihalyi). 12hrs will pass by quickly and you’ll be able to regain some of the focus you are missing. If you can’t find this at the place you work, maybe consider changing workplaces.
You just need better planning. I posted the method that I use and I am really productive [0].
Since March 2016 I do 9 hours a day, 7 days a week. That's 63 hours a week.
Of course, I would love to do more but I end up being frazzled.
For 5 mins a day, try to focus and observe something. The standard is breathing because it's easy and always existent. Just watch your breath go in and out.
You can try guided meditation too, like Headspace.
Meditation can feel extremely difficult at first. You'll feel that the time is better used for something else. Your mind will bounce all over the place.
It will most likely get even worse too at the beginning. Some people actually scream.
Some places will suggest doing 30. I don't. We've seen people start at 30 mins because they think it's easy and all, and then they go into the office all stressed out.
I find the minimum dose is 7 consecutive days. Try to do 5 minutes, 7 days. Once you can do 7 consecutive days comfortably, you can start adding more minutes in. Increase the dosage gradually, especially if you encounter significant discomfort.
Then, the key thing to focus on would be to get work done:
1. Begin your day by making a long-list
2. Prioritise - 1 do-or-die task, 3 must do, rest are good-to-have. It is important to prioritise, else nothing gets done. Better to accomplish 1 thing every day than attempting 10
3. Once prioritised. lock down. Break down into tasks that take less than 30 minutes. And spend time on a task 15-30 minutes until it is done. Everything else can wait
4. Rinse and repeat
I have suffered this same problem before. Allocating too much time for tasks - getting nothing done. Key is to focus on doing things rather than allocating time. Hope this helps
While on the internet I use a pomodoro timer and website blockers to try and keep me focused on the task at hand, both of these are pretty crude solutions that can sometimes even be counterproductive though.
(shameless plug: I'm working on an "intelligent" website blocker in my spare time - https://awakely.herokuapp.com)
Modafinil/provigil can be helpful if you have a clear and inflexible plan for what you need to do during the day. I find that they often make things even worse once I do get distracted though. And they are more conducive to productivity on simple repeatable tasks than complex, difficult tasks that require context switching to learn new things. I take 100mg first thing in the morning and drink extra water to prevent headaches.
Making lists of what you want/need to accomplish that day can be useful for keeping you accountable to yourself, but again they often work better with well-defined tasks, otherwise you end up with your original list half-complete because you had to do other things that you couldn't anticipate and the day ends up feeling like a failure.
A low-carb/slow-carb diet has been helpful for keeping my focus more constant throughout the day, especially a fat/protein based breakfast. Some advocate a ketogenic diet for this too, although I haven't gone that far myself.
Getting 8 hours of sleep each night makes a huge difference. Strangely, I actually find ~4 hours sleep makes me more productive than 6, this certainly isn't good long term though.
As others have said, your biggest issues is probably just attempting to work 12 hours a day in the first place - most people just can't pull this off on a regular schedule.
I don't think you will fix this problem instantly. It will take time, so don't panic. Also remember that some days you are gonna be less productive than usual, don't be hard on yourself then.
It happens because a nature of current state of an internet. You can try to limit your working time to a few slices of 4-6 working hours. Also try to replace short and vivid content like any news, videos to the weekly newsletter, books, tv series and so on.
I used to have the same issue and it's solved now by removing news, youtube, all social networks, junk spammy content with ads from my life.
Terrific recent podcast discussion here on this very subject with Dr. John Ratey. Authored “Spark: The Revolutionary New Science of Exercise and the Brain,” > https://findingmastery.net/john-ratey/