How do I make cookies that don't be come hard as a rock the next day? What am I doing wrong?

You may want to check the accuracy of your measurements, particularly flour. The best way to measure flour is by weight--if you don't sift the flour and plop it into a measuring cup, it is easy to use too much.

This thread may help you:
http://food52.com/hotline/23069-what-preservative-can-i-add-to-cookies-bars-to-keep-fresh-longer-even-well-wrapped-freezing-ta

Don't over mix your dough. Use a food processor to cut in butter just like you would for a tart and then pulse liquids or eggs just until the dough begins to form. Lightly hand knead to incorporate the remaining flour. You can also store cookies in a plastic container with a slice of bread and that will keep them soft.