How can i solve my problem?

i just finished school, and i have 2 weeks to study for some official exams.my chances of succeeding are nearly 100%, but still i decided to work very hard to get a "good" or a "very good". i am very wise for my age, and very, very good in scientific subjects, but i have this very big problem of laziness... i've had this problem the whole year. i sit and write a schedule for let's say a week, but when i actually sit at the desk to start work, i just cant start! there's like deviations in my mind. i think that im not in the mood and go play xbox nstead :( so what i want to know is a way in which i can actually do some work, have about 8 hours of lesson done daily instead of 2. keep in mind taht when i used to study with my mom, i used to learn more than 8 hours a day! i used to complain, but at least i used to get the jobb done every time. also, im 15, and as i said i am wise and i think im strong enough to go over this. thanks!!