I get the impression that it's eating away at you quite a bit - "I need to tell someone and talk it through - but who?".
There's no right answer to this. Those who are saying, "tell no one" would probably have a different view if the question was, "I used to use prostitutes a lot when I was a lot younger - should I tell my girlfriend now that we're engaged?".
To those who say "tell all", then I'd also say that too has issues.
The comment I've found least helpful is "Why would you burden your future husband with this knowledge? How would it be fair on him; just to make yourself feel better?"
But, that's the problem. You've got issues with your past that haven't been resolved yet and they've now started to pick at your mind. Well, they've been doing that for a long time and it's not getting any better. You'll end up just telling all at some point when it gets too much for you to handle.
Hattiep got it right - you were abused. I'd sit down with your other half and tell him that you've got things in your past that you haven't resolved yet and that you want his help in dealing with them. Explain to him that you were a kid when it happened and you should have been under the care of the State under S31 of the Children's Act - which would have given you protection and it failed miserably in its duty. You need to explain that you were a16 year old child and on your own.
Ask him if you can share the past you don't like or is he going to say you're still on your own...