Severe pain in legs while standing

i'm 42 years old and for over 10 years I'm suffering from a terrible pain in my legs. This has caused a lot to my professional carrier and none of the doctors whom I consulted could diagnose it.

When I stand/walk for about 10 min, a severe pain with burning sensation in both the feet happens. The pain is such as if the legs are not able to take the body pressure. The pain subsides when I sit for a while. There is no pain while sleeping or when I wake up in the morning. Everything is normal except while walking / standing.

Few months back when I had check on blood, the vitamin D3 level was extremely low (less than 5) since then I'm taking D3 supplement. Along with that B12 and also B-50 complex. On top of this, I'm also taking Magnesium Citrate. All of them for about a month now.

Two weeks back, the Neuro specialist could not find a reason for this. Well, he has put me on Gabapentin 100mg but again no improvement.

could anyone PLEASE PLEASE help me to get a relief by directing me how to diagnosis this problem?

add to the above: i'm 5.6 ft and 62kg weight.

Thank you.