Erm...
I don't know if you are actually serious?
Because if you were doing 15 hours a day, although it is possible, it means that if you worked from 6am to 9pm straight (without eating, breaks, being on the student room etc) you would only have time to sleep...
If you took breaks, you would literally have 6 hours sleep (I have not read the whole forum so I do not know what you do) and that is not enough.
If you are being serious, chill.
Instead of doing well, you will fail, because you will be too tired and will not learn anything. Sorry to be blunt, and good luck. ☺