Hello there,
@shivajikobardan,
What you're experiencing is completely normal. I love learning new things, but even I can get tired of it. Don't worry if this happens to you, especially after something as heavy as an exam, as sometimes the next thing will come along and it will feel like a chore.
If you are studying try to space things out: study a few pages a day, revise over it(flashcards, writing notes, speaking to yourself, etc.) as you will give yourself more room to breathe and rest before doing all of this again the next day. Make sure to stay in touch with your teacher too: they can help with whatever you may be struggling with.
As you claim this has been going on for a long time, keep an eye on your thoughts and emotions: a lack of interest in your hobbies or discipline could be a sign of mental health problems, including depression. To combat these problems by yourself, the least you can do is keep yourself healthy(exercise and good diet - these essential tasks of everyday life can help to develop a healthy mind) and entertained(relaxing music, your favourite film/TV show, etc.) and keep in regular contact with friends/family. Of course, these only benefit you short-term: the longer you feel this way, the more you are better off seeing a doctor/therapist about your problems. Just note that feeling like this can in some cases, be only temporary(SAD - seasonal affective disorder("winter depression")?)
Last but not least, you may just not be enjoying the subject you're currently on or even the field of study in general. If this is the case, you may want to evaluate over your studies so far: what have you learned? What else is there to learn? Is there something here that I'm still interested in? Asking these sort of questions may help you in understanding why you feel this way about your studies.
I hope this helps!