Yes, other applications, even running in the background, use memory.
You might want to check with a local computer service to see about increasing the memory.

(Luckily, my last computer did not come loaded with a bunch of crap, but prior ones did. Being a programmer, I despised that and always removed all of the "junk" programs.)

Think of RAM as a mixing bowl and programs running as the ingredients. You can only put so may ingredients in the bowl before you need a larger bowl.

Answers to this question will come easy to the experienced, but might be difficult for others to discover. If your machine runs Windows 10 I am pretty sure it has enough built-in RAM to perform still photo editing well. Video editing is much more RAM intensive and might be pushing the limits. I can thing of only two ideas that you can use on your own. Before you seek help you might want to make sure you are running only PSE while editing. Running other programs at the same time could create problems. You should restart your machine at least once or twice a week. A restart might close any background processes.

Use the task manager (control-alt-delete) to see what background processes might be running. CPU use should return to 10% or less when the machine is idle. Background processes could be consuming all available RAM, and might reveal a virus or malware. You might be able to "end-task" running processes, but well written malware will start right up again, or you might close something your PC needs. Write down the name of the process then Google it to see what it does and if it is malware.

If none of this helps you probably need experienced, but not necessarily paid, help. Start with a more experienced friend or relative. If you do have to seek paid help you probably will have to spend less than $100. Any more than this and you might want to look into a different consultant or a new PC.