can tough brisket get softer? I am cooking a brisket in a pot with water and vegetables, on a stove top. 

I meant to leave it on a slow simmer, so that it would get very soft. About 2 hours after I started cooking it, after having left the room for a while, I found it in a rolling boil. 

The meat was no longer pink or red, but a grayish-brown well-done colour, and tasted cooked but was a little tough.  Will it get softer if I cook it longer on a slow simmer?
Well there is good news and bad news. The good news is, if you keep cooking it, the proteins will eventually break down and the meat will get softer. The bad news is, the boiling for so long may have spent the goodness of the meat.

More good news however. The meaty goodness has likely been transferred to the broth. You could convert your dish to a small bite stew, and no one would be the wiser, and the broth will impart some great flavor to root veg you might add to a stew. (Potato, parsnips, turnips, carrots, etc.)