It would be a lot happier in the ground if its possible - the stems should be rather thicker than that although this is a very dainty variety. Did you prune it this spring? Paniculata pruning is different from mophead pruning as they flower on the current year's growth. So in late winter or early spring prune it right back leaving a third of the length of stems as an absolute maximum - I'd take more than that from most of them (do this every year), and if it has to stay in a pot tip it out when its dormant and see if the roots fill the pot. If they do give it a slightly bigger one, in any case an some fresh compost.