Magnesium oxide is not appreciably absorbed... it doesn't work.
It is a good laxative tho...because it remains in the bowel and works that way.

I have a magnesium thread that explains it:
http://neurotalk.psychcentral.com/thread1138.html

Since you have had this for a LONG time...I'd try the acetyl carnitine and others (omit the P5P for now), for 2 months or so to see what happens.

When you use fixed combination vits, you cannot control the doses very well.

For example:
Thiamine B1 works only in really high dose 300mg a day. Or its newer version benfotiamine. The 100mg in your b-complex is just not enough.

I would cut back on the b-complex to 1 a day.
I'd increase the B12 to 5000mcg a day for 2 months.(on an empty stomach)
I'd try acetyl carnitine(1-2 grams starting at 500mg a day and increasing slowly) and CoQ-10 (300mg of quality enhanced absorption) for 2 months.
I'd change the magnesium to a better form.
I'd add 200mg of OTC thiamine 100mg daily in divided doses.
I'd try r-lipoic acid 100mg a day

This would cover mito damage, and metabolic problems with
dehydrogenase enzymes.

I'd get tested for Vit D levels...which when low contribute to chronic pain.
See this new video just released as to how important this is:
http://neurotalk.psychcentral.com/thread92116.html

The acetyl carnitine, and CoQ-10 will cost you. Most of the others are not so expensive.
But if you really want to seriously try supplements you will have to do MORE.

If you have an kidney damage, or other health problems you should consult with a doctor to see if what you do is safe.

But at this point I think 100mg of B6 is unnecessary and may be problematic for you.