Making fully cooked meat (pork) less tough
It sounds as if you have meat that has been cooked past the point of well done, so that all of the proteins as as contracted and tough as they are going to be, without the couner-veiling benefit of having collagen conversion.

Your only recourse, which may or may not give you results you find acceptable, is to resort to mechanical tenderization.  Your best bet is to grind it in the food processor.

Since you mention making dumplings, a healthy portion of cooked cabbage, mushroom, or other vegetables in the filling will also help get a more appropriate texture.