I think getting your B12 and Vit D tested is your next step.

Metformin can result in lower B12 levels in many people.

Get the numerical results, and don't accept "normal" from
the doctor. B12 lab ranges are reported in an outdated format in many places, and very low numbers are given "normal" status.
The new therapeutic guidelines for it is a minimum of 400 pg/ml in the US. Low B12 will damage nerves over time.

Also there are reports that fixing low Vit D in diabetics improves PN. So you need to have those two tests run.

Type 2 diabetics may burn out the pancreas over time, and therefore need injected insulin. The fact that you did not respond to metformin is not normal. It is a possible flag that you are really type I. Type I can occur in adulthood, and I've met 2 people who were misdiagnosed in their 40's and 50's.
Do you know your A1C number? This can be a better indicator of type I or II for some patients.

Nerve damage can also come from drugs. Statins for cholesterol and certain antibiotics can do this. Here is an informational thread on the topic.
http://neurotalk.psychcentral.com/thread122889.html

So right there are two other reasons for PN besides diabetes.
Cipro, Levaquin and Avelox are given too freely by doctors, and some people develop nerve damage from them. Statins are common too, and can be very damaging. Often these triggers are ignored by doctors.