Sudden weight loss implies low fat diet. If you do not provide
essential fatty acids daily (good fats) the nerves cannot maintain themselves. Nerves are mostly fat in composition.
There is even a medical term for this: "slimmer's paralysis".

The nutrients specifically to repair myelin are B12, folate, B6 and
fish oil.

There is an RX version of activated B's called Metanx. If you decide to use it, take it on an empty stomach.
There are people who require the active forms because of genetic failures of conversion in the body.

When nerves wake up from being numb, they often tingle. Also when cold and then warm up, they tingle then too. So keeping a log maybe helpful for you. And using tingling as a guide is not always accurate. Keeping your feet cold like you are will result in tingling later, as the blood flow improves again. It can be a pattern of your circulation and not a symptom per se.

Blood work for serum levels of B6 and magnesium are only valuable when they show very low levels. Otherwise, the ranges are not reflective of much. Better tests involve intracellular testing, so consider those. Spectracell labs is one type.

Sometimes people with progressive problems and strange reactions to supplements may have gluten intolerance...meaning the gluten peptide has escaped into your system, and is causing various autoimmune/inflammatory problems. Here is a link to that:
http://neurotalk.psychcentral.com/thread1872.html

Fixing the gluten problem by going gluten free, has helped some people who have come here over the years with PN.
Gluten intolerance may be Celiac (genetic testing) or acquired later in life due to leaky gut, without the genetic markers.

I'd start on the Kefir ASAP. YOu might find a considerable improvement.

500 is not a sterling result on B12... as Dr. Snow, in his research found people with neurological damage at those levels:
this is a link to his paper on it:
http://neurotalk.psychcentral.com/post698522-70.html

You could take 1000mcg (1mg) daily orally, on an empty stomach and see if you can get that level up closer to 1000.
The higher the level, the more successful the B12 can travel into the spinal cord and brain. You will want methylcobalamin oral.

You can test yourself....either borrow relatives glucometers, or get one yourself. You might be very surprised at what you find.
One Touch ultra mini is free---but you have to buy the strips or get a doctor to RX them to you, for insurance. This probably would put you on "diabetic" diagnosis. This can help if you ever need diabetic support, special shoes etc. But it also labels you
for pre-existing conditions. There are so many people with insulin resistance....who don't know it. Another test is a fasting INSULIN...and if this is elevated, that points to impaired glucose tolerance as well. The beginning of diabetes is often silent, and there may even be very low blood sugars that are missed indicating too much internal insulin secretion in the beginning. Alot of lows lead to nerve damage, since the nerves are not getting enough glucose to run the energy mitochondria.

So...
Get some Kefir... at least 4 ounces a day.
Metanx RX if you prefer (has active methylB12, active B6 and methylfolate in it)
Fish oil
R-lipoic acid 100mg a day and Benfotiamine 300mg a day.

Eat magnesium rich foods, like beans, nuts (almonds), etc.
And see what happens in 3 months.
Consider going gluten free, do the research on that.
These are the main items. Some of us do more, some less.
But most people will respond to this list in some way.

Make sure you have normal kidney functions, and that your doctor
approves.