While complete vulnerability is lovely, it creates a complicated dynamic for people who have experienced sexual abuse, since that kind of abuse creates a strong connection between being vulnerable and being violated.
IL2M, I know what you mean. I was sexually abused as a kid and raped at 12. And given the statistics, I'm sure there are many other people who post to this forum who have similar experiences.
Those walls are pretty necessary for a while in order to create safe healing space for yourself. But yeah, they can be tough to break through when they're not as needed anymore. There's no one real way that works for everyone and different people react differently to such trauma. There are so many variables in that. (I first coped by not being sexual with other people at all for a good period of my life.) It does sound like you've actually already broken through a huge wall by having such a deep emotional connection with your fiancÚ that makes you feel safe. You also have the advantage of awareness.
All I can say is that it's work I have to revisit in my life from time to time, but I'm ok with that. And a good catharsis during sex can be really freeing and amazing