Sorry this is long, it resonates hugely here too. You're not alone.
I also told/created a stupid lie. It was life changing making a rotten situation far worse.
I wasn’t encouraged to have friends at school and was supposed to keep myself to myself. At junior school I was alternately either supposed to sit on a wall away from the school and wait to be picked up two to three hours after finishing, or if she was going to be later, make a long journey back and let myself into a grubby damp basement to wait, as I wasn’t allowed inside the flat alone.
In that unsupervised time, I’d secretly made a friend and been to her house. She was nice and her parents where kind to me and their basic home and life seemed like a fantasy dolls house to me.
But then she expected to come to mine, which I hadn’t known was normal.
Problem was my mother was a) working in a school so her reputation trumped everything, and b) an extreme hoarder who’d lost control of a monumental hoard and everything had turned to squalor. She also wasn’t coping and there was a lot of physical and emotional abuse going on.
Friend started making it clear not getting a return invite was a snub.
Desperate to keep her, I cleared up the cat crap from our basement, (cat lived there) swept out as best I could and lined up some chairs, (there because they had no seats and woodworm) put some material over them as a bed, a box for a table, and invited my friend round to play. 
She told her mother who apparently cried over the conditions in ‘my room’ before telling school. All hell let loose and I was a nasty little attention seeking liar who’d upset and hurt the only people who’d ever been nice to me, they where to good for me, and decades on typing that still makes me cry. I was told they couldn’t sleep, trust others, all sorts.
School told everyone to stay even clearer of me.
What was done to me for at home for it all, is beyond explaining.
The truth was the conditions I’d invented where a big step up.
I actually had no bed, sibling had one but I wasn't allowed on or in it. I had a very tiny space that I could barely turn in to exist in, ‘play’ in, and wasn’t allowed to touch almost anything in our home, and the damp, leaks and rotting material and dead creatures upstairs made my cleaned dressed up cellar seem a presentable ‘home’ to my young mind. I wasn't attention seeking or intentionally setting out to lie.
The 'lying’ to my friend and damaging her and her mother, and the need to protect my mother at all costs, altered the course of my life for the worse. Investigation over it lead to CSA and in turn a need to make sure that didn’t wreck the perpetrators life, and it all slid further down and down from there on.
We make childhood mistakes, sometimes big, and sometimes they alter our whole lives.
I was brought up as liar to protect adults’ behaviors, and it never occurred to me that my own free will lie could be so much worse.
I was accused of being a liar so much, while still being expected to constantly lie to protect, and ended up a ‘proper’ compulsive liar and a thief. (again, started with stealing to cover up what was going on)
You got help, and I’m so glad for you. Yes, children making up CSA causes issues for those going through it, but it’s not the real big issue around CSA - which is adults abusing children and jumping on anything they can find to cover it up. Please hear that you aren’t responsible for that, OK?
Tell fiance the simple truth. Lies, including by omission, often require more and more omissions or lies, especially when someone who knows your ‘secret’ uses it, as I suspect just happened to you.
At best she's tone deaf and dangerous, at worse seeking power and control over you.
If he’s decent he'll recognise that a silly little girl did something stupid and paid a high price and he needs to 'disapprovingly' say 'it's a taboo subject here' if workmate/wife ever raises it again.
If he doesn’t, then how ever painful, you actually don’t want to be marrying him.
Don't let your past be too big a deal in your future. I too would give you the biggest hug if I could.
Yoksha 
In the unlikely event Denise B (as would have been back then) reads this and recognizes it: I’m SO sorry, Ironically, I just wanted to hang on to your friendship, nothing else.