HI Bex,
I know what you mean about feeling old. I feel like I went through old age in my late teens/ early twenties!

If you can't convince your doctor to do the saliva test you could call one of the labs and ask them which doctors close to you use their service. One of the labs in NZ that does the test is:
http://www.pharmaceutical.co.nz/

The test is a little expensive but could be worth it if it gives you somewhere to start.

All the best!
Cara