Think deeply what makes you happy? Or the happiest moment of your life. For me it was when I tried number theory(prime number theory), computational complexity. What about you? So if that doesn't make up your mind. Expose your self to the subjects in university level ...how ? Just Google & YouTube is enough. And then try to see yourself through it. But mind your own decision it doesn't have to be the fact that you know the subject very well nor does it have to be because of any fame, money, reputation or your desire to prove your self to others. It sometimes happen that you are studying something with on reason and enjoying it whether you came up with the answer or not (whether you get it or not) that's how you know you fall in love with it. You might also stumble upon 2 or 3 subjects and you shouldn't be worried. You can double major or like you know.... your first degree and 2nd degree ...