Why can't I stay asleep after regaining full consciousness?

I know there have been similar threads but I didn't think they mentioned specifically what I meant to ask...

Anyways, I woke up about a couple hours ago and I remembered myself and knew that I was sleeping, and like usual when that happens, I felt excitement that now I could control my dreams. I began to fly and open my eyes wider and try and change my speed, the size of the moon, make hills and fields, etc., but then I felt it all fading because I must've not put enough focus into my thoughts, and then I half-way woke up.

When I was half-awake, I knew that if I stayed still and imagined I was still flying, I would feel many vibrations through my body and I would be there again, but it didn't work because I felt a strange fear, like for some reason I always feel when trying to go back to my dream after half-waking up.

I am just wondering why I have trouble staying asleep after I remember myself in my dreams.

I'm not sure I entirely get what your saying.. From your ld count I see your not completely new to this, you've maybe had more than me, im guessing it could be to do with your REM cycle just ending, that's quite common, either that or you tried to do to much in the dream, some advise not go flying and changing things so much straight away, stabilizing and taking in the dream lets it last longer.
Next time try looking at your hands if things start to fade, the dream should stabilize, then repeat.

Sounds like a balance issue. Putting too much focus on your thoughts and not the dreamworld is a good way to find yourself in bed.