Mantras are not magical (well not this kind), so you need to understand what you want and give it time. Mantras about dreaming set up a background thought that should subtly resonate in your mind after you have stopped repeating it consciously. Picture a kid wants to box but flinches, he may repeat "I will not flinch" a few thousand times over a few weeks. This drives into his mind that thought. In learning to lucid dream when you do this as you fall asleep hopefully the thought gets stuck in your head like a song can. You are not really doing anything wrong, just stick to it. Try very simple phrases.