can tough brisket get softer?
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.)