I completely understand what you must be going through. Nevertheless, there are solutions to resolve this dilemma too. And in fact we usually come across students around your age who face similar issues while making their career decision. So there's nothing abnormal about getting confused :)
In order to resolve this dilemma -
1) Read up more about career with Math and Biology and also with commerce. So that you are well prepared with the necessary information. Jot down what interests you the most and make a list. Know everything about those options in terms of career prospectus, remuneration,career path,colleges,benefits of the career etc. One place where you will find all this is
2)Once you done with the external information, lets look at the internal information i.e. information about yourself. In order to make a wiser career decision you need to know more about yourself in terms of your interests, aptitudes and personality too. You will need to give a career test for this which will give you an unbiased picture of what you are like.
You will also find a sample report here which will help you know how your report will look like. Once the test is done you will get your report with 2 to 3 important career recommendation for you.
Further if you have any queries on that and report you can anytime come down for a counseling session with the report, and the counselors will be glad to help you out.