It really is hard to say. I know that in a large group I do have trouble taking in info. A lot of that is because I am concerned that I may ignore someone. Yet, it is almost impossible to take in all that info at once. And yes, the fact that you now have more responsibilities and are concerned about them, may be leading you to self doubts. At 17 you have not had a lot of time to learn how to cope or deal with this stuff (some people never learn how).
You say that you are naturally talented at math - my question would be "how do you do on math tests". It is difficult for a person with ADHD to concentrate the full time while taking a test. If you do poorly on tests, yet feel that you know the material - than that could definitely be a sign of ADHD. I have always felt that academic frustration, - ie. you know that you are smarter then the person next to you, but they always do better on tests - was a pretty good indicator.
Frankly, I was always depressed from Sept to dec., but that was because I was a teacher.
Any way, I think at least part of the problem is that you are in what is probably the hardest part of high school. You are aware of the tremendous pressures on you, and maybe for the first time, you actually are worried about them.
I think a partial key to what is going on is how well you are doing to what you think you can do (if you are realistic). I am sorry I can't tell you more, but there is just not enough info. I would suggest that you don't keep this bottled up inside of you. Talk with a counselor, or maybe a teacher and let them know what you are feeling.
And, of course the forum is always open. Good Luck.
I'd have to query Sandman2's statement that "It is difficult for a person with ADHD to concentrate the full time while taking a test", as it really depends on what kind of test. If it's a subject that you really enjoy and are naturally talented in, and the test itself isn't TOO boring, even someone with ADD can find it easy. It's a commonly held misconception that people with ADD can't concentrate at all; generally they can concentrate very well when they find something intrinsically interesting!
It does sound, from what you describe, like you might have ADD (I'd guess Inattentive type since you don't mention anything that indicates hyperactivity), but there are other possible explanations, so you should really see a doctor to help narrow it down. Although ADD is thought of as a childhood condition, it is not uncommon for people to find that it actually gets worse as they reach adulthood due to the increased level of responsibility. This tends to be especially true of women, who often have much less noticeable symptoms in childhood then suddenly realise they have huge problems in adult life (I'd count myself as one of those!!). I'm guessing from your user name that you're male, but I suspect since women tend to be more likely to have Inattentive ADD, it might be true of males with Inattentive ADD too (although I have no solid evidence to base this claim on).
The symptoms you describe sound very similar to mine. I have to make clear, though, that I don't actually have an official diagnosis, because I live in the UK and it's really hard to get a diagnosis as an adult (my doctor didn't really seem aware that adults could even have ADD!). However, I'm 99.9% certain that I do have ADD (I'm a psychology student so I'm not basing this purely on a few websites I've read or anything! I've read the dianostic criteria, many academic papers & several good books. Also, I have discussed my problems with my educational psychology lecturer who agreed that my problems did sound very much like Inattentive ADD.) You don't say where you live, but if you live in the UK I suggest you see someone about it ASAP as you'll find it much easier to get a referral while you're still classed as a child. IIRC, there are only two clinics in the entire country that deal with adult ADD, and they have huge waiting lists. So don't leave it until you're 18!!