what is your exact Os version? exact computer hardware? what exact Ae version? what have you got in your setup? show us screenshots. bare in mind that Ae is designed to give real-time playback of cached frames. it could be that you are running low on available RAM and Ae gives you what it can cache to memory.
more about preview here: Preview without rendering in After Effects
here's a video for you: Uninterrupted and simplified preview in After Effects
I should have plenty of room. If you look on the photo of AE opened you can see the scene I am working on is the tutorial about the spaceship. If you look in the little red line in the timeline you can see that it goes only to 5 seconds (each time the distance varies - this was longer then most). It then loops back over that 'green' area - over and over again, so I never see the completed work.
I am using the Essentials format. And, Yes, I have seen both those tutorials but they don't quite hit the nail on the head for this concern.
Oooh, I see you have only 4 GB RAM. That's barely enough to run the OS and After Effects at the same time. I'm not surprised your previews are so short.
You need to procure more memory, ma'am. 16GB is a good amount. Don't blanch at the number; many users have 32 or 64 GB. But 16 does the trick without breaking the bank.
this is your problem
your system has a very low RAM. this means less room for caching frames. these are the system requirements
you are below recommended level so don't expect too much of a performance with your machine.
there are ways to get by. lowering the resolution pop up menu and other things. read all about them here: