So I don’t quite understand the copy paste function.

So I recently noticed that when I go to copy something, after I paste it whatever I copied seems to immediately disappear or sometimes it won’t let me even paste what I’ve copied. Why is this? If there not a timer on what you copy? Because I tried to copy paste something extremely important. And because of frustration I accidentally deleted what I wanted to copy and paste. Now it’s gone forever. Why can’t the copy stay on a timer instead of immediately disappearing? Can someone please help me.