Does anyone else have trouble singing over instrumentals?

I genuinely feel like my voice has gotten to a great spot. I can sing all of the pop songs I listen to at this point. But when it comes to making my own instrumental and singing over it, my technique goes out the window. I strain, yell, and lose confidence.

It is the most frustrating thing. Is this a common issue? I have no idea how people free style over instrumentals without having a "guide" in your head. Is just practicing over and over really going to fix that?