It is easy to spot a teacher who loves their work. They seem to emanate contagious energy. Even if it on a subject like advanced calculus, the subject comes alive. If you dont love your work or your subject, it will come through in your teaching. Try to figure out why you feel so unmotivated and uninspired. It might have nothing to do with the subject, but your expectations. Adjust them a bit and you might find your love of teaching come flooding back.