You've gotten some good advice in this thread already. I know I don't know you, but I'll chime in with some thoughts.
Your post makes it sound like you're a very serious student, which is great and I commend that. It's all about balance, though. I agree about human contact being a must. Whatever form that takes: hanging with friends, playing sports, being in a band, church, boyfriend/girlfriend, etc. It can be tough spending too much time alone or just focused on studies. You might get burnt out. That may not apply to you, you may have tons of friends already and plenty of social activities, but it's worth a mention.
I will say that depression doesn't just go away by ignoring it. Sometimes talking to someone qualified (a school counselor, therapist, etc.) can be a good idea. There's no shame it in and it can help you focus on who you are and where you want to go in life. Just to get answers and bounce ideas off someone can help. They can also help you figure out what makes you happy and how to pursue those things.
Not that you have to share anything too personal, but is there any unresolved conflict or loss in your life that you haven't dealt with? Maybe a family member who is ill or recently deceased? Disapproving parents? Any internal conflict like that? Just curious if there might be something that has your mind preoccupied. Sometimes it can be a short-term circumstance that's got you down instead of a general funk or rut.
Judging by your profile, you're still very young and have an awesome future ahead of you. You'll never be as young, carefree and healthy as you are during 18 to 30, so try and make the most of those years. Trust me, life's too short to spend too much time being depressed. Sometimes I know you can't help it, but if it extends past a short season of depression, it can be pretty detrimental to your life. You deserve to be happy and enjoy life. Sounds cheesy, I know, but it's true.
Anyway, whatever's got you down, you're not alone in it. Find out who your support system is and use it to your advantage. Lean on the ones you care about and who care about you. You don't have to go through it alone.
Best of luck and I hope things improve for you.