Personally, I think that's something that should be talked about BEFORE an engagement. I think it's her right to know. Obviously, you can't go back in time though.
As to how much to tell: I'd say just keep it basic to begin with and tell her that you're not a virgin. Then allow her to ask whatever she wants. She might not want any details, then again she might want a lot.
If she feels misled, and I could see why she would, that could be another issue to deal with on top of this. Things like that can shake some trust.
Regardless, do the right thing. You already know what you should do it seems..Just do it. You said your relationship has survived a lot of things already, so hopefully that will help here by having that history.
As to how much to tell: I'd say just keep it basic to begin with and tell her that you're not a virgin. Then allow her to ask whatever she wants. She might not want any details, then again she might want a lot.
If she feels misled, and I could see why she would, that could be another issue to deal with on top of this. Things like that can shake some trust.
Regardless, do the right thing. You already know what you should do it seems..Just do it. You said your relationship has survived a lot of things already, so hopefully that will help here by having that history.