Can someone explain what I'm doing wrong when cooking with cream?

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When I cook any recipe that has to do with cream, like creamy mushroom sauce or some Indian cuisine. I am faced with this issue: Now,watching tutorial vids. I notice that the cream is heavy. but it's more liquid than a paste.. but the only kind of cream I am able to find in grocery stores is the paste-like heavy cream. So, I tend to lighten it just a little bit with water to make it easier to cook with. like the one in the videos online. Now,what happens is that eventually. the cream in the pan is more oily than creamy.. meaning that the cream looks like it's separated by oil and it becomes weird and smudgy. I don't know whether that is the result of my lightening it with water. or is it that I have left the cream heating in the pan for too long?Or something else?