I've been in a similar sort of situation lately, a few months back it reached the point when I was on the verge of a nervous breakdown, but fortunately I got help and sorted it out (mostly)
Firstly, I'd suggest going to your G.P. about the physical symptoms; not having to worry about them would be a good start. As for the problems causing it, you need to tell someone, either a best friend or understanding parent (who won't mind if you end up in tears, and will give you a hug and get you a tissue), or if you don't think you can face doing that in person, you could try the Samaritans, either phoning them or sending them an e-mail (I found that really useful, in that you can list everything that's bothering you, which feels good to get off your chest in one go - I think it's samaritans.org, could be wrong; google it)
In terms of practical things you can do, try setting aside time for yourself everyday, where you don't worry about anything else that might be troubling you, and just relax; laze about in front of Neighbours/ have a nap/ read a book/ dance around your room in your underwear/ bake a victoria sponge/ knit a really long scarf/ whatever, or better still, get some exercise (a jog in the park if it's sunny or something). Also, cut out the caffiene; it only makes matters worse (and with regards to dark chocolate, don't medicate yourself on it. if you're going to have it, try and get the best stuff you can afford, without loads of additives in it; a little of the good stuff goes a long way). Eating healthily helps, even if its just having some fruit instead of those cookies while watching Neighbours...
Sorry about the mini-essay (I'm currently procrastinating from writing a real one), hope some of that is useful!