hiya
I have experianced this since a teen. and its sleep paralysis. This can occur as you fall asleep or as you are waking mainly but can occur in the middle of a sleep. Its not very nice at all. Most of the time people experiance, total inability to talk, make a noise. Move any part of your body. But the most prominent thing is the feel or dread, a malignant figure , presense, or apparition of a figure that makes the experiance alot worse.
If you read up about it, you will see alot of information. It was discovered by a neurologist and is a problem many have. I know from personal experiance if they are getting very frequent, and causing alot of distress as yours seem to be. You can see a doctor/GP who can help. My mum had these for over 20 years till she discovered what they were she had then 4+ times a night and was afraid to sleep. He sent her to a specialist and was given treatment which helped.
Mine are less frequent mostly a couple of times a week. And i know what it is, and i breathe slowly and try to disract my mind, as the more you fight it the worse it gets. Over time i have learnt to get through them. I suggest reading up best you can.
Good luck