TL;DR: Multitasking is not what you are after, you should instead look for a system like GTD.
I am finding it difficult to concentrate on different things.
Do you really need to concentrate on different things at the exact same time?
Whenever I do something I just get concentrated on that and all other things fall apart.
This is something that we call "tunnel vision", and is completely normal if the task requires your attention.
If you don't give the task your attention, the task will not be productive: The multitasking myth...
Sometimes these things get mixed up and get messed up.
This is easily solved by not multitasking, but rather schedule your tasks based on context / time and energy which is what is known as a Getting Things Done system. A system like GTD simply works because it takes any thoughts on your brain and keeps them away in lists of tasks, which you will progress one task at a time instead of all tasks at the same time.
Like during exams I will just think about the hobby or even get interested in a single subject while I have other 5 more subjects to learn.
That's a matter of scheduling time, so that they don't interfere with each other.
This is exactly my problem. I dont want to do things simultaneously. I want to divide my limited time to different activities. But I get hooked on one activity and try to do it better, leaving behind all other things. I want to divide my concentration and time equally between these activities. – 0xakhil – 2011-07-19T14:48:40.633