Re: I Hurt My Wife
I agree with Faeluna here too. If I had known about it in advance, I probably would have kept some distance from this other woman too - just kind of awkward.
I would say a big part of the issue for me would be this obvious 'hiding' of the fact that has been going on - maybe not on your part (if you assumed she had known), but on the girlfriend's side. If it had come out from her as 'omg, i remember when i hooked up with him before you guys started dating', or if it had been 'well, i don't really want to talk about it, talk to him'. That's a difference.
Yeah, it's a pity that your wife is in this situation now, but you haven't done anything wrong imho (as long as you did not try to consciously keep this info from her). Your wife will have to make some decisons about her feelings regarding the girlfriend.
I would suggest supporting her, telling her you're sorry that she found out this way, but honestly believed she knew about it (what did she think you two were doing when you dated?). And tell her that you want her to feel okay with everything and you're behind her whatever she decides (i.e. breaking contact with the girlfriend or the circle of friends, at least for a while).
Even though it was over a decade ago, your wife is just now finding out about this, and some of her reactions will probably be as if it just happened a few months ago. If/when you all get together, she'll probably be watching the interactions between you and the girlfriend with a magnifiying glass.