If you find that the mantras and visualizations are keeping you awake, it's fine to stop doing them so you can get some sleep. Do them for about 10-15 minutes at least, and if you haven't fallen asleep then just stop, get comfortable, and assure yourself that the best thing for you right now is to fall asleep. Even if MILDing is not the last thing on your mind as you fall asleep, the effort you put in will still help tremendously. And as important as doing the visualizations and mantras is, it's even more important to fall asleep - obviously you can't get lucid in a dream if you're not getting asleep to have the dreams in the first place.