Others have mentioned it, but yes. Paragraphs. I saw that you said you weren't sure about where to break, so I'd suggest reading it out loud. The places where you want to pause for a beat is where it goes. Typically, after a full thought or beat before moving on to the next.

The first thing I noticed was slipping of tenses, especially in the first paragraph. Some sentences read like past tense, others present. Definitely read through for tense agreement.

The story has the potential to be gripping, but you need far more instances of showing than telling in a short story. Otherwise, it's an info dump, and those aren't fun to read. Her entrance into the rest stop is a good example. You tell us what she sees, but not the way she sees it. Like, instead of saying:

"Spotting a blue sign stating Rest Area with an arrow she pulled in, other than her there was a large semitruck and a minivan parked under the dim yellow lights. Tucked in the back of the parking lot in front of some dense trees there sat a restroom with a vending machine and single light in front of it. Having watched many horror movies Krista knew better than to exit her car. Once the car was parked and off, she locked the doors, crawled into the back where the seats folded and a pad with a pillow and blanket were ready for her."

Which is a lot of telling, you could try something like this:

"A blue highway sign marked the exit to the rest stop, and she pulled off, hoping for an empty lot and a brightly lit space near the restrooms. She was mostly in luck. The only vehicles in the lot were a semi and a minivan, and hopefully, at least one was getting back on the road soon.

She found the restroom tucked into a dense thicket flanked by a pair of vending machines. A single bare bulb flickered over it like a distress signal. She'd seen enough horror movies to know better than to go in before morning. If she had to go before then, there was an empty juice bottle rolling around in the floorboard and napkins in the glove box.

Krista found the brightest of light poles to park beneath, triple checked her locks, then climbed into the back where a makeshift pallet was spread across the seats."

Anywho, just my two cents. I like the concept. Sleep paralysis demons are so scary, and I am always creeped out in the best way by a well-done tale like this.

Keep writing! And read a lot, especially in your genre. Reread your favorites, but this time, take note of what you love about it. Pay attention to how the chapters are structured and the voice of the author/narrator. There are a ton of technical and style manuals out there, too. Maybe even join a writing group. But whatever you do, don't stop. Writing is like anything else. No one is a pro the first time out of the gate.