Is it normal that i have a phobia of my fiancé leaving?

Before I say anything I need to disclaim that he has already voiced that he has absolutely no intention of leaving me. I have no intention of leaving him either.

Basically at one point in our relationship about a year and a half ago me and my current fiancé split for a week.
It wasn't a bitter break up at all, in fact it was about as pleasant as breakups get. Hence why we were able to get back together so quickly. (It was basically a crappy friend convinced him that I was making him unhappy, which he quickly realised was a manipulation because the said friend didn't like me.)

The issue is, even though we are engaged, I have this huge irrational fear of him leaving me.

I get so nervous every day about it. He has told me countless times that he promises that isn't going to happen. Even going into logic to try and get me to stop worrying, saying "that ring should be all the proof you need that I have no intention of leaving."

But yet the fear is there. He can't say anything to get rid of it and god bless him he tries, I just can not shake off this fear and it hurts me so much that I even worry about that. Before we broke up initially I would never consider the idea of us breaking up. I just wish I could get back to that mindset. IIN?