Your GP sounds rubbish unfortunately. So many of them are when it comes to chronic pain and pain relief.
It took me 6 years to get my condition diagnosed (which I only got by moving surgeries, where they still tried to fob me off at first), at which time two of my joints had deteriorated to the point of no return - and I'm now waiting for joint replacement surgery (for 2 out of 5 joints.)
I am in my early 30s.
What dose of codeine are you on? Does it help?Did she say why she wanted you to reduce it? Have you got a hospital consultant?
I find that the GPs tend to just want to get everyone off the opioids to get their numbers down - that's why they shove people onto ADs and nerve meds.
Amitriptyline only helped me with sleep, I was still out cold in the morning (like the hangover grogginess) and really struggled to get going, I also couldn't drive.
I just kept going back to my consultant telling him how much pain I was in, we trialled tons of meds before we got to SR morphine, oramorph for BT pain and paracetamol, plus duloxetine (another AD) before bed.
It works, enough for me to get about day to day anyway.
I'd ring the surgery again and ask for an appointment with another Dr, just get them to explain. If the amitriptyline isn't working, they'll have to change the med - but you are going to need to kick up a bit fuss, unfortunately.
Wish you well 