If you heat your balsamic glaze for too long, it is going to turn hard when it cools. To fix it, you may be able to reheat it and add some liquid to it to thin it out. In fact, just try a teaspoon of water. That should work just fine. Simply add a teaspoon of warm water and stir briskly until it loosens up. If that is not enough, add another spoonful until it is the consistency you like.