How do you know if you have made a right choice?

Follow your heart, not your thoughts. Once you do that, you are always correct.

You have much to learn, my apprentice...heheh

Choose what you enjoy first and then move down the list of enjoyment with your number one being the one you most like and the higher numbers the ones you like less. It is important to enjoy what you do in life anyway, studies should be the same as I see it.

You don't know for sure.
You go with what is right at the time,and sometimes, time changes things so it might not be right forever.
You prioritse,you go for it, and if things change you have learnt something along the way.
Yes you only live once so you need to cram as much in as you can,so so what if you end up studying 101 things? they are all strings to your bow and it is all experience.
Youll eventually find your way even if its through process of elimination
I think life is a learning curve and you don't have the ability to see your whole future,so you just have to roll with it if things don't go to plan,and have confidence in yourself ;o)