Growth under my thumb nail

I have a very painful growth under my thumb nail. It is there all the time and appears at the top edge of my nail as hard skin. I cut this as it causes my nail to split, but my nail then becomes infected. I have tried cutting the nail down the middle (very painful!) but cannot get to the root of the problem. I have had treatemtns for fungal infection, tried fucidin for the infection but nothing helps. I saw a dermatologist who did not know what it was. I asked could he remove my nail to see what it was and try and cure it but he said the nail wouldn't grow back properly. I am now totally frustrated as it is there constantly, can you offer any advice??