You can't divide it into smaller bushes if it is growing on a single leg of course so I guess it has thrown up a lot of branches from ground level. Some of them may well have their own roots, but you would be safer layering some of them and starting again. Do this by pegging down some of the lower branches to help them to root into the ground. making a small shallow cut in the part that will be buried often helps to stimulate root growth.
They may have rooted by next Spring or may need a little longer. Then dig them up, plant out and train them to the shape you want.

Trying to get a spade or fork into a mature bush to divide it would be a horrendous task.

The large bush can be heavily pruned however, if that would help. February is the time to do it. Take out about a third of the branches at ground level, concentrating on the oldest ones. Remove any branches that cross over each other.Then shorten the leaders back to the size you want and shorten all the small branchlets that come off them to leave no more than half a dozen buds on each. Then give it a good feed of potash fertilizer. You can shorten all the branches a little now if you like but do the main job in Feb. A very prickly job but worth the effort.