my homemade pastrami is tough

Did you make the pastrami in a smoker, or do you mean you cooked it in a slow cooker? Pastrami really needs moisture, even while smoking, to help it stay tender. I generally put in an 9x12 pan of water beneath my cured brisket in the smoker, just to make sure there is enough moist steam during the smoking process. If you don't, the meat can dry out and be tough. But pastrami is a rather tough meat anyway, which is why it is always sliced really thin to eat. For your current cut, perhaps slicing it really thin and reheating it with steam may help. It might also be due to a shortcoming in your corned beef. Not all of them are great, and if you don't start with a good corned product, there's not much you can do to improve it. You want to get a cut with a nice fat cap to start with, as well. Not enough fat on the cut and it's going to be dry and tough pretty much no matter what you do. Also, when you cut and reheat, always do that with steam or at least water in the pan.