Why can focus for a long time on games/books/tv but not studying?

It sounds like you have poor self control. Have you considered trying to break your work up into milestones, and then taking a short break to do something fun after each milestone? You don't necessarily need to stare at a maths textbook for 4 hours straight.

Okay I think honestly most people have this problem to some extent. I know I certainly used to. I've even been known to spend exam time (such as GCSEs and A levels) actually daydreaming and not concentrating. Needless to say, not good.

The problem can be a number of things though; there might be too many distractions around you, you might be frustrated with the subjects you're doing, you might not have the best revision technique to keep you interested... You get the point.

I found that setting myself allotted amounts of time for specific pieces of work eased me into being able to concentrate better. Also when you're working turn off your phone etc so you'll be more focused on the task in hand. The only exception for me being my iPod, which I'd play concentration or classical music through, as it seemed to help as well.

Also, and this isn't easy, you may need to change your attitude to work. Instead of seeing it as something that evil teachers or lecturers set you for a laugh, see it as something that's going to help you get the grades you want or even just help you write essays more clearly or have better exam technique. This was the game changer for me.

Out of interest what subject/subjects are you studying?
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At the moment I'm studying for some Maths tests in late October. But I had similar problems on other subjects. It's the same at work, I have a friend who would probably not notice a naked model walking down the office corridor yet for me its almost impossible to put all my attention in to one task. Even with no physical distractions my mind wanders all over the place.

There is certainly something with deadlines focusing the mind. In April/May I was lot better than the preceding months, if there is some way to mentally convince myself that I have a constant deadline looming I would get more done but not sure it would be good for my mental health!

I was exactly the same, and when it came to cramming etc I ended up with good grades.

It sounds like you need to get into the habit of catching yourself out, as it were, when you daydream. This is something I took from meditation, which sounds silly but it works.
When you mediate you're told to keep brining your attention back to your breathing and notice when your mind wanders. The more I did this, the less frequently I found my mind pulling my thoughts away. So now I do exactly the same with work. The minute my mind stops focusing on the task in hand is the minute I mentally shake myself and return to the task. Maybe give it a go?
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I have tried mindfulness a few times, first it was it help my sleeping (same problem of mind going all over the place) but in the end I got much better results from listening to a podcast of something like science or radio4 and have slept really well since.

It's certainly worth trying some kind of focusing technique though, my some kind of picture that reminds me to get back on track. I am trying different types of music, classical can work but it's a bit slow so trying something a but more like dance.