Why did my cream sauce separate when I added the cheese? It turn into a soup like liquid with curds!?

If your cream sauce was too hot when you added the cheese, that can happen. The sauce only needs to be hot enough to melt the cheese. If you added the cheese while it was being heated on the stove, that can be the cause. Sometimes, especially with the drier and older cheeses like parmesan, you need a bit of extra fat in the sauce to start with. You also want to shred the cheese yourself. Packaged shredded cheese is coated with starch and other things, which tend to cause problems when used in sauces. It also helps sometimes to add a pinch of dry mustard to your sauce before adding the cheese. You don't need more than a pinch, and it usually doesn't change the taste. Mustard will help act as an emulsifier to keep the liquid in the sauce playing nicely with the cheese.

Did you make a roux...melted butter, flour, milk or cream? It’s the base for cheese sauce that I have great success. I remove it from the heat and add the cheese. Then return it to the lowest heat until I see one bubble, then it’s ready.

This might give some ideas 😄
https://food52.com/blog/14876-kitchen-rescue-how-to-fix-a-broken-or-curdled-sauce