I particularly like Jake's suggestion of joining a class, if it works in your circumstances. One reason for your slump may be incipient boredom with whatever your doing, and working with a group can help break the cycle.
If you don't mind elaborating, where is your mind generally focused during performance of the form? Are you trying to recall body positions, form sequences, fighting a battle, meditating, checking out the Ten Essentials? I find that the more appropriate and challenging my general focus, the more satisfying my training is.
I have had one teacher that approaches T'ai Chi partially as an excercise in mind expansion. He advocates all sorts of practices to explore the full implications of the T'ai Chi circle, such as visualizing colors in conjunction with certain postures to observe their effect on the body. I personally find that I am still quite challenged working on basics, so my visualizations are usually more geared to basic skills. Nevertheless, I do find it stimulating to glimpse how wide ranging some of the principles can be.
One challenge I could propose off the top of my head is visualizing you are doing the form on a surfboard, on a windsurfer, in a canoe, or on a water-slicked sheet of ice. For this to work, you must actually feel the thought in your body, not just imagine it in your mind; and you must maintain the visualization during every moment of the posture transitions.
With this visualization, can you maintain your root and balance well enough at every millisecond of the form so that you do not get dunked or crack your skull? Do you still have power to launch a technique (fa jin) at every point of the form? By the way, the idea is definitely not to test your balance per se, but to see what parts of your body and mind feel important in the process.
I find that the windsurfer visualization radically focuses my mind on one meaning of "distinguishing full and empty," since I must be ready to pass my weight back and forth between my feet from millisecond to millisecond, even if the swell of the "waves" does not cooperate.
Let me know if any of this helps.