Sometimes I'm so scared of losing myself I'm 18

A college freshman, and sometimes I feel so scared of losing myself.. I used to be so deeply into astronomy and animals and encyclopedias of every kind. There was a time I used to know every plant's name, scientific name, special facts and it's purpose. I used to read fictional story books of 500+ pages EVERYDAY in one sitting if I wanted to. Then everything changed. As I grew up, I realised studying for school, doing my homework is important, so I did that. It reduced the time I spent on my books which were not in my curriculum. Still I tried my best to achieve things, studying for mbbs exam, not getting mbbs, going for another college. And here I'm doing Bsc Nursing and wondering, do I not have the right to do what I love, just because I'm not rich enough? I cannot even read 300 page books in one sitting currently.. it's strange. I can barely process things in my brain now. I've become strange. Astronomy and plants excite me less than a good night's sleep. Strangely I feel melancholic, is this it? Is this what life is? A meaningless void, where you are squandering from place to another, losing your hobbies and chasing after money?