Dan has got the rotation of sails right, but i would say that after 2 years, you should change the boat. F18 is a fast developing class and it is a mistake to enter in to it thinking that your boat can be up to what ever level it is when you buy it, for any longer than this.
So spend as much as you can on a boat but dont expect it to become part of the family. If this scares you a bit, stick to one design sailing.

paul