if its at 80% usage at start up, look in task manager/performance tab/memory and see how much is showing in non paged pool.
Try a clean boot and see if it changes anything - make sure to read instructions and make sure NOT to disable any Microsoft services or windows won't load right - https://support.microsoft.com/en-au/help/929135/how-to-perform-a-clean-boot-in-windows
All it does is stops programs loading at startup, it doesn't delete any, Easy to reverse.
if clean boot fixes it, it shows its likely a startup program. You should, over a number of startups. restart the programs you stopped to isolate the one that is to blame.
if its just in the game, its a big game. It wants as much ram as you can give it. Since its not finished it is no where near optimized yet. If they ever get around to that stage, last I saw they were telling people off for expecting them to live up to promises.