In my experience, the first LDs I had where I tried to control the dreamcontent usually led me to wake up very soon afterwards. Maybe because this is just too exciting and the level of lucidity wasn't high enough to stay dreaming under all the excitement.

My advice:
Upon becomming Lucid, just sit back and observe the dreamworld all around you. Not only will this be a magically beautifull experience, it'll keep you in the dream longer than when you would try and rule the world. Do that for a while to increase Lucidity. Then when you feel your Lucidity is much stronger, try subtile dream control, such as flying, Morphing Objects, letting DCs float...etc Don't focus on changing the entire Universe around you since that might be too mentally exerting and exciting and lead you to wake up.