Try breathing slower than you normally would, and deeper than you normally would, and concentrate on exhaling completely before you start to inhale again. That should allow you descend, slowly. Remember that you descend by exhaling, completely, then waiting a second or two before beginning to inhale again. Also remember never to hold your breath, whether after inhaling or after exhaling. Slowing down your breathing could help you to relax.
If you cannot relax, then you may need another instructor. Some personalities simply don't mesh, especially if this instructor reminds you of a former gym coach whom you did not like very well in the past, etc.
If you still cannot relax, even with a new instructor, then you may not be ready for scuba, yet. Then try snorkeling, or freediving, before you come back to scuba again for another try.