Trust the middle aged ladies on this-GO!
The downside is that no one will have your back, you'll have to figure everything out yourself, and you won't have anyone to talk to. The upside is that you'll learn how to do that on your own. You won't have to compromise and miss things you want, or have to see things you don't care about. If you get lost, no one can blame you. No fights.
Pat's right-watch your alcohol intake. Stay in the central part of the cities. This first trip, alone, is not the time to stay in a dodgy hostel an hour outside of London. If that's what you have arranged, change it. You don't need to run for cover when it gets dark, but neither should you stay out until 3 am. You can party with friends when you get back. Concentrate on seeing the sights.
Keep both an electronic and paper copies of your travel arrangements. You're the one in charge of yourself, so be in charge. Often kids your age have come to rely on immediate help being just a cell phone call away. Rely on yourself-get a paper map, keep a business card from your hotel, don't be spacey and lose stuff.
Take care of your belongings, because no one else can loan you money. Get a money belt and keep your passport, extra cash, and credit/debit cards in it. Keep the cash you'll need daily in a more easily accessible spot, Read the info on this site about money belts. In fact, read a lot on this site. It will give you more confidence. Don't be freaked out by tales of pickpockets, etc. They are just cautionary tales that you should learn from. Be prepared, not scared.
You could cancel and may wait 30 years for someone to go with you. If you wait, you will always regret it. Go.