When you think about lucid dreaming it can often trigger an expectation of a lucid dream, the desire for a lucid dream comes into the mind and you in a way hope for one to occur. But this added excitedness can lead to a cancellation since it you are too excited to actually have a stable awareness within a dream. Though it may seem hard to not think about it, just try to tell yourself of some of the cool things that could happen in your dreams tonight and let your mind slowly drift away. I had the exact same problem as you have, but that is what i did to get rid of it, it may work for you, but its worth a try. Try your best not to expect things since it will most likely get you excited which isn't always good for lucid dreams.