Windows complains it's running out of memory with 6 GB free

My PC has 16 GB which I think should be enough to run a game and a video editing program concurrently but Windows complains it's running out of memory anyway with 6 GB free. I'm not making Pixar animations so I don't want to hear about how studio machines have 128 GB of memory. Hitfilm Express is using less than 300 MB at the moment and the game is something like 1.7-2 GB.

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I had the same issue a while back. Like you I'm running 16 Gigs of ram.
To check what is going on, Just open the task manager and look a the memory column, Then scroll down to Runtime Broker.
I'll bet you that is the percentage number is between 90 and 99% . If it is, You got a memory leak. You will have to disable apps running in the back ground one at a time. When you find it the Runtime Broker percentage will drop to around 10%.
In my case onedrive was where the leak was. I don't use it, So I disabled it and I was off and running again...