Two questions

Recently I've been doing the same thing everynight,but something happends that I don't understand.So I lay down, put my hands on my chest, stay completly still, and inhale slowly then exhale fastly.Once I don't have to think about my breathing I focus on what I want to dream about.After a while my body gos numb and I can't move.Why does this always happen?
My second question is; How do I gain control of my dream?The other night in my dream I found out that I was dreaming. So I do a reality check (I can breathe through my nose.).I try and walk off a edge and fly, but as I start to move everything beigins to go black. After the image fades I wake up.This always happens.

You were entering sleep paralysis. We enter it every night unconsciously to stop ourselves from actin out our dreams. You probably woke up from the excitement of knowing that you're dreaming. Just stay calm and touch everything around you to stabilize, along with using all of your other senses before going off.

THe first questions, it sounds like sleep paralysis.
About control, you might want to stabilize your dream first.

Kushybear has a great idea with using all his senses to stabilize a dream. That's my favourite technique also! Take a few moments when you get lucid to really take in the dream. Look around in amazement at this projection of your mind (which is another one of my favourite things to do).