Why do I involuntarily stop breathing during my sleep?

I go to sleep without thinking about anything before I go to bed. Then, in the middle of the night, while I'm still asleep, I can't breathe at all (usually the stopping of breathing coincides with a nightmare I'm having). I subconciously stop breathing all together like I'm holding my breathe underwater or something. Even as I am sleeping, I keep telling myself in my mind to breathe, but my body won't listen to me. I try my hardest to open my eyes, or yell for help, but I can't move my body at all as if my motor skills have been paralyzed or something. It's as if my body is fully sleeping, but my mind is fully awake, as I clearly know for a fact at the moment it's happening that if I don't breathe, I'm going to die. I really fear that I will die in my sleep this way someday. What is wrong with me, and what can I do? I feel like I'm being smothered with a pillow, or drowning underwater. Please help me. 10 points for best, thorough answer! Thanks in advance.