Hey Shxh,
That's not a silly thought at all. I certainly felt quite similar when I was your age. It's been seven years since then, so it's really easy for me to look back and think '
there was nothing to worry about', but I do understand that's a bit difficult in the moment. I'm sure 30-year-old-Caleb will look back and think the same thing about 21-year-old-Caleb. It's something that'll get easier with age. However it's certainly not uncommon so don't think it's weird!
While it's great to have a future goal, at the moment all you can do towards that is trying to set consistent study habits — pay attention in class, do your homework, and ask for help when you need it. However there's not much else you should be doing at the moment. Year 12 is a
long time away, and by the time you get closer to it, you'll have lots of guidance from your school on
how to prepare and subsequently get into medicine.
In the meantime, I'd like to think enjoying the moment is a bit more important than studying your ass off. This can be, well, whatever you want. Do you like reading? Go read a book or fifty. Sport? There's basketball hoops everywhere. Want to be a bit rebellious? Well, I won't tell. I definitely studied in high school, but when I look back at high school I think about the fun things I did rather than how I prepared for my future course/career.
To put it in another way: getting life experience is going to prepare you for a career as a doctor in ways your textbooks won't. So please know it's healthy to have a balance
With regards to stress management, PhoenixxFire gave some good advice — a counsellor, teacher, or doctor will be able to help out in ways that we can't.