As the previous posters have said, you have unfortunately signed a contract, which is a binding agreement. So they are entitled to enforce payment. You probably have a few options;
1- Eat humble pie, show them a breakdown of your finances to demonstrate you really cant afford it, and ask them to let you off entirely.
2-If they won't do that, ask if there is a cheaper membership plan they can switch you to.
3- Ask if they will let you buy out of the contract at for a reduced fee, or pay off the amount over a longer period.
4- Ask if they will let you find someone else to take over your contract, which some places will do- but they onus is on you to find someone.
5- If you have good reason to think you will be better off in say 6 or 9 months, ask them if they would consider putting your membership on hold for a period, and then picking it up and starting paying for it again when that period expires.
Above all, be polite, and accept that at the end of the day this was your fault, not theirs, and make sure you are seen to be making very effort to come to a satisfactory resolution.
It would be worth putting your various offers in writing, even if you discuss them in person. That way, if you fail to come to agreement, and default, you will have something to show the court that you have tried to work through it if they try small claims or such avenues.