Welcome to DreamViews and congrats on becoming lucid! Control just takes some practice and experience. For changing things behind a door, I suggest imagining whatever it is before opening the door. When you go to open it...open it very slowly and imagine what you will see as you open it. If you see it isn't working, close the door and repeat. Just slowly open the door, imagining the desired outcome, and be confident in your approach. Don't worry, control will come. If you struggle with something, don't be afraid to ask questions, and just keep practicing. Control is very flexible in dreams and what works for some may not work well for others. Get creative with it and try out many different things to see what works best for you. Good luck and again, congrats!