You are still in the head-spinning stage. You've just learned something that you never imagined would happen to you, and you are probably visualizing the worst case scenarios. Yes, the fear leaves. Many cancers are driven into remission; it is no longer a death sentence. Once you have a handle on you treatments, a direction, you'll be able to start focussing on the knowns instead of the unknowns. Hang in there - you'll breathe again. Having a plan gives you some control.
However, if for the time being you need sleeping meds or anti-anxiety meds to help you cope - well this is what they are for: that short term stiuation that just has you in its grip.
I never thought I'd get over the fear; never thought I'd laugh again. I've done both, and continue to live. I'm 5 years out from a stage 4 diagnosis, and it hasn't been easy.