I had exactly the same problems when I was 18. I was the valedictorian and constantly worried about my AP tests and getting into the ivies and UC schools. I didn't know if my problems with my moods were stress-related or symptoms of an illness. I suspected that I had Bipolar, but I didn't get any confirmation until I was 20.

It took so long for me to find out what was wrong with me because I never asked anyone for help. I didn't see a psychologist until after I'd had a total meltdown during university. So the best advice I can give is to find someone you can talk to -- a trusted friend, a teacher, or a psychologist. Go to a general practitioner and tell her how you feel. She can give you advice, help you figure out what's going wrong, and suggest possible courses of action.

If you have a positive relationship with your dad, then you could try talking to him. I was pretty much spent my teenage years being raised by a guy who has a Ph.D. in Chemistry. He's tough and holds me to impossibly high standards, but he's also come to understand that I'm far from flawless. If your dad loves you, then he's going to accept you and care about you and want to help you even though you're not perfect. I don't think he'll be disappointed or upset with you just because you need help. He'll probably think that asking for help is a sign of courage and maturity.

Also, good luck on those AP exams. If I remember correctly, they're in May.