The fact is most students have come to associate pain with studying. It is viewed as a means to an end: a bitter medicine that needs to be ingested if one is to earn a good income some day. Problem is the thought of a six figure salary some time in the future is actually a weak motive for putting yourself through twelve years of school and four-plus years of university. You need to enjoy what you're being taught and take pleasure in studying it now. So I'm saying a better future shouldn't be the only reason you study. Before you roll your eyes at my Utopian vision, give the following three points some serious consideration:
#1: Studying is a privilege, it is not a sacrifice
Remind yourself every single say that there are millions of people with no access to books, no chance of going to college and many who will never even get to go to school. Ideologically education is a right, but practically it is still largely a privilege based on where you were born and the income of your parents. Studying therefore is not a pain or sacrifice, but an honor.
#2: Pick interesting subjects
If you still find studying painful, you're studying the wrong thing. You should not need any external incentive to study; the thought of learning something that interests you should be reason enough. Depending where in the world you live and the type of school you attend, you are allowed to pick your subjects somewhere between the eight and tenth grades. Like I said, the thought of earning a 'big' salary someday is usually not enough to compensate you for years of toil learning things that don't interest you. And then one day when you do earn $350,000 from a stressful, boring job, you will feel as though you're making $15 million too little. You can't study for a career just because there's industry demand for it; you have to pursue subject matter that interests you and find ways to make it commercially viable.
#3: Mix your study routine, work it into your stride
Okay, so you're studying to pass an examination and inevitably you will come across a section or subject that you don't like but have to learn. Provided you are studying a course that genuinely interests you on aggregate, you can mix the unsavory sections with the fun ones to ease the pain. When you study something only because you were told it's a good career though, then there's nowhere to hide. Mixing salt with vinegar will not produce a sweet result.
Study for as long as your mind permits. On some days you may be able to go for three hours straight, while on other days your brain may insist that you take a break every 30 minutes. Sitting at your desk trying to force information into your mind while it isn't receptive is wasteful. Take a nap, go for a walk or have a fruit to give your mind a breather. Students too often create rigid, intensive timetables with little room for deviation and rest. Be organized, be disciplined, but be human too.
You need to turn education into a pursuit of discovery and yes, fun, rather than one of incentivized obligation. Knowledge is pleasure.