Yeah, that's when it becomes scary, when you're aware of the fact you're dreaming but totally unable to break out, constantly in the cycle waking into dream after dream after dream, that's the hell that feels endless. It scares me so much every time it happens(though it happens less and less now) yet in retrospect seems silly.
I learnt a bit about sleep paralysis in my parapsych lecture(remember to buy that book i mentioned elsewhere btw!), I think a very prominent feature was the pressing down/difficulty breathing thing, but I've never totally had that(semi, i guess) and so I wonder the 'range' of SP, and wonder how far it truly differs from false awakenings where you know you're asleep(sometimes I'm both dreaming/moving a dream, yet aware I'm dreaming and can feel but not move my body, despite very great effort). Tis fascinating!