1)Whenever I do my work, I get bored of it in 5 minutes and find it hard to persist.
To rectify this, perhaps consider whether the work you are doing will actually assist in either enhancing your understanding or your marks-establish goals for each study session, establish short and long term goals-you can use academic marks or ATAR, you don't necessary need a specific career goal to motivate you, maybe listen to some concentration music to prolong your concentration span-over a period of time, you can increase your concentration from 5, to 10, then gradually to an ideal 40-45 minute time span. To persist with something-you must attempt to find a passion or inspiration for the subjects you are doing-thinking about what you want to get out of this year is very important.
2)I tell myself I'd devote an hour to a subject but I go do something else halfway.-
it doesn't matter, because if one hour doesn't work, then do it in 30 minute span, when you get bored of a subject, study for another subject, alternating between different subjects is a way of maximising your study efficiency and minimising your boredom, make sure you are learning in a way that optimises your level of retention-i.e. if you are not a visual learner and don't remember by looking at a page for half an hour, then consider recording notes/essays on an audio recording device and remember things that way, or you might find someone to explain concepts to, or pretend to explain concepts to in front of a mirror
3)if the time, I find questions repetitive and get the idea after one or two questions but it seems that it just doesn't compute in tests.
-Try to ask your teacher for harder questions, try to find harder textbooks which has less repetitive, more complex questions that requires an integration of your knowledge. To understand how to do a question is not enough, the whole point of sometimes practising questions repeatedly is to make sure it becomes a reflex action-essentially it allows you to do these questions very fast and with very high accuracy in tests so that you can devote more time into questions that requires more lateral thinking-and hence gain more marks. Getting an idea is not exactly your goal, your goal should be to get each question 100% and know how to approach a never before seen, difficult question using fundamental principles that you learnt in your easier ones-try the questions under exam conditions-practice makes perfect
4)I don't know where to get my motivation from since I don't really know where I'm headed in the future.
Have a watch of the video,
http://www.youtube.com/watch?v=_pEBz6PzF50 -re-watch it if necessary to get short boosts of motivation
-a lot of people your age probably also don't know where they are headed in the future, but the concrete fact is, getting a higher ATAR will allow you to have more choices of going to as many university courses directly as possible, and this is super important when you don't know what you want to do-and you want to make sure that at the end of the year, when you must decide what you want to do, your ATAR can help you to get to that degree in less time. Try to find a source of motivation that comes from yourself, not built upon your friends/parents/peer expectation
-Just remember one more thing, in the long run in life, in the HSC year, to be smart is not enough-you must harness your talents and quick learning capacities to enable you to reach your maximum potential, seek to be a hard-working person, and smart is relative, whether one is smart or not depends on who you compare that person to, however, hard-working is less relative-just ask yourself this one last question-imagine you are at the end of your HSC year, do you want to say you have completed this year with the best of your effort and have no regrets or that you have completed this year with regret that you haven't tried your best. Perhaps try to compete with yourself-when you are able to excel/improve against your previous results-that is a smart as well as hard working person
I truly hope you will soon found that source of motivation and change your attitude around to 'I am going to work hard'-with a new goal in mind, anything is possible if you set your heart to it. Hope this helps and all my best wishes for you doing brilliantly in your HSC year-a year where you will have no regrets
(It is never too late to change)