Wow you have more discipline than me. I would be a miserable 
if I only masturbated once a month.
I have almost the same issue as you though but unfortunately am far less chill about it. I think councilling looks like the first answer but I suspect it won’t change anything.
Could be several reasons.
Yes she seems concerned over becoming pregnant.
What was her childhood and teens like? Was she brought up by parents who never spoke about sex or thought it was dirty.
Has she ever tried to be sexy and for whatever reason, you turned it down, because that can really have a negative effect on how she may now feel.
I personally dont think you should separate, not if the rest of your relationship is ok, but you both really need to speak to each other