You just have to get used to previewing portions of your comp. The whole world does it in AE -- it's nothing new at all. Read up on setting the AE Work Area.
You can also preview at Half, Third or Quarter resolution, use the Region of Interest if you don't need to see the whole comp window for previews -- which gives you more RAM to work with -- and you can preview every other frame.
Sounds like you are trying to do a short movie in AE. AE is for creating shots and short sequences. Most of my AE comps are 7 to 10 seconds. You edit in Premiere Pro or your favorite NLE, you composite in AE.
If you really need to preview a longer comp in AE then you can set the Composition Panel resolution to Auto, the Magnification Factor to 50% or less or you can set skip frames in the Preview Panel, or you can set the Magnification Factor to 25% or manually reduce the comp panel resolution. The other option is to buy more ram.
A good practice for anyone doing compositing and motion graphics in AE is to break their work down into shots, make each shot that needs work that cannot be done in a NLE into an AE comp. If you need to put together a sequence limit the sequence to sentence or phrase of copy. When you set up the motion and animation start by doing what I like to call a pencil test - no effects, just motion and usually at a smaller magnification factor or if I need to see the motion at full size, lower resolution. I am borrowing a workflow set up by animators more than 100 years ago, quick pencil test first, then turn that into a movie and check the timing and action, then ink and paint and final photography.
When the pencil test is working the way you want it to add your effects and final polish and preview a few of the critical frames at full rez and full size to make sure you have the look you want, then send the project to the AME to render a DI (digital intermediate) or a test (mp4 for sharing) and move on to the next shot or sequence in AE while you are rendering. I don't know anybody that ever gets anything done in AE that tries to run a full resolution full effects preview that is more than a few seconds long before then send the comp out to render. It's a total waste of time.
When you get your shots rendered to a DI cut the final in a NLE like Premiere Pro and fine tune the cuts, the timing, the audio and then do the final audio mix and color grade if your project requires this. Working on a 3:15 movie in AE and making changes to that project would be a nightmare and completely inefficient. As an editor and especially as a compositor you have to learn to visualize in short sequences.