i tried to explain that I never intended for her to find out
I can't see that as helpful. So it would have been OK if she just didn't find out?
I messed up a couple of months ago. A friendship, nothing sexual. But it hurt her a lot because of past betrayals. I also didn't know what to do. In the end it worked out with a lot of communications and more honesty. (not that I was dishonest but previously I moderated what I said in order not to hurt/trigger her as she was/is dealing with some horrible issues. It's complicated.)
For you I would say your craiglist days are over. No not just the personals, anything on craiglist. No buying or selling. The site no longer exists for you. And tell her that. You need to so in the future you can honestly say that you have never been on the site since.
Tell her where you are going and where you have been. For a couple of reasons. So that she knows and doesn't have to guess, and to communicate better with her. Don't tell it like a report but more of a sharing. "hey I was at xxxx and you wouldn't belife what I saw ..."
Get interested in what she does. Spend more time treating her as the special woman that she is. Take her out. Go on dates.
It will take time.