I'd like to join Sophie_M in welcoming you here to the support forum. I'm sad to hear you are sad but know many people here, of your age, younger and older, have gone though the same thing and come out OK. There have been times when I've been sad for weeks and could not say why or what to do about it.
Living in a great home, going to a good school and having no obvious hassles is not realy relevant -though you might think it was. Seems silly doesn't it? Sadly anxiety, and depression too, takes no notice of such things. I thought my life was all one could want, did not help.
Before we go further may I ask how you get on with your family, for instance do you parents pressure you to do well, or give you a hard time in other ways, do you have brother or sisters that make things difficult or others at school who do?
I'm not sure there is an instant answer where you can say "That's the trouble", it is more a question of getting help and your anxiety -plus any other conditions - under control and made better. Then the sadness will stop by itself. I got absolutely nowhere without the correct professional help. Then things improved.
Can I ask if you are being treated for aniety ATM? If so I guess you need to go and tell them it's not working and think again, maybe see if you have depression too as Sophie mentions. If you are not being treated see if your parents will book you a long consultation with your GP and say exactly how you have been feeling. Your parents can be asked to wait outside, it is your appointment.
Kids Help Line (phone or chat) on1800 55 1800 or
would be a good place to talk things over and get some advice, they can be a real comfort and know what they are talking about.
Sophie's recommendations above are particularity good in Reach Out too. and their Forum is OK.
Do you have to face all of this alone? Is there anyone you can just talk with and get care and a shoulder to lean on. Isolation simply makes things worse. Could you talk to your mum do you think?
I hope to hear from you again XP