If the problem really is insufficient light, pruning won't help in the long run as it will regrow in much the same way. You could think about using some subtle "hoop style" staking to force it into a more upright growth pattern - this often works pretty well in low light situations.

Thank you maingrower, that's pretty much what I thought about the pruning.

Can you move it to a sunnier location? That's the easiest option I can see from here. That's not much growth for five years, even though it has blooms.
If it's located in an area that's a little too shady now, it's going to get shadier every year unless major changes are made. You either have to move it, or limb up some trees, or even cut some down to get the results you want.
Mike

It is getting enough light to set buds, but not enough to prevent the floppy growth. It isn't doing too bad. Once it gets to the size you want, you can start breaking off the terminal vegetative buds each spring so it maintains that size. The existing branches will get much stronger and the plant will fill out.