I am 17 and currently in the same situation looking for cheaper car insurance, a few tips: add experienced drivers to the policy, it shows that the use of the car will be shared reducing chances of an accident (even if the added driers will never drive it) fully comp is often cheaper than third party as it shows that you care more about the car again reducing the chance of accidents. finally, bump up the voluntary excess to max, this should bring the cost down slightly. Good luck, I know the feeling of getting nowhere but have faith, it will go down, just keep trying different things on quotes.
Benny