Welcome to Dreamviews! It's fairly common to wake up early* during your first few lucids. This will improve simply by having more lucid dreams, luckily. Have you tried stabilization techniques (rubbing your hands together, spinning)? How do your failed attempts go (do you remember what your goal is?)?

*You could be truly waking up early, due to excitement or inexperience (as your brain may initially have trouble being fully conscious and dreaming at the same time). Another possibility is that you became lucid at the natural end of your dream (again, you're likely to be more conscious closer to waking).

When does season 2 for Dollhouse begin, by the way? :x