my mop head hydrangea is soo heavy the heads hit the ground

I wonder if the variety you have was originally one of the types developed as a cool room/house plant? Did you get the original from a house plant dept in a garden centre? These types are notoriously floppy I have found if successful outside.

If it is one of the many normal type varieties available for outside, then it needs to build up a more branched woody structure to help it support the flower stems.

Cutting back (especially if hard) is only going to produce none flowering stems and its far to late in the year to be cutting a hydrangea back. Anyway, soft growth produced by cutting back now will still be left at the tips of these new stems and they will probably suffer quite a lot of die back over winter from their tips back. If you can bear it, remove all the flowers but leave the stems in place. Next bud burst 2010, take the stems back about a quarter, no more to a good pair of buds. Hopefully this will be the start of forming a stronger support.

Thank you for your advice. I first took a cutting of this plant about 6 years ago from a garden that had not been tended for many years. The hydrangea at this old property was over 5ft tall and upright almost like a hedge. It was beautiful. My cutting grew and was magnificient in a large tub for a few years until it became too heavy.
I will do as you advise because I would be sad to ruin this plant.Many thanks for your reply.