The mental universe is a dark labyrinth.
Every field of study is a new corridor of the labyrinth.
When you learn, you light the way through these corridors with candles.
Most of the candles stay lit, and you can revisit the lit caves that you have illuminated before.
Sometimes, a candle will burn out, and you will have to restudy it -- relight the candle.
As with any adventure, the first candles you are given excite you, and you begin to look into many different corridors, back away, choose another, until you run out of candles and have to take a break.
But there will come a time when you know you have gone deeply enough into the labyrinth that you will have to decide once and for all which corridor to travel down.
Well, that was my analogy for the insurmountable obstacle that is choosing a major and a particular field of study in a time when you feel so curious about every area of knowledge that making a choice seems absurd.