Most Helpful Girl
If you loved her so much then you shouldn't have done it in the first place. You need to learn how to communicate and be open about things that your not satisfied with. Once you lose trust from a women it's really hard to gain it back cause that's all she will be thinking and it will be miserable for you both. Take this as a lesson learned.
Most Helpful Guy
First, most importantly, you're lucky none of the girls on craigslist have responded, I'd say the majority of these posts are done by professional scammers - it worked on me once, so you should be happy it didn't happen to you. Second, I'm not sure how to help you with your girl, but seeing as she's the unforgiving type, it could be like talking to a wall when you're expressing genuine concern - if I were you, I'd find a mutual friend, say an uncle in law, or her cousin that likes you, and have her be the middleman/woman.
Next, I came to notice that you fantasized. This doesn't make you an idiot, it means that the relationship failed to meet your expectations. If you're wandering around, maybe it's for the best. I personally think it's plausible, and forgive my ignorance, that you did what you did because you're dissatisfied with your relationship, albeit being in love with your gf. I'd say, instead of getting back with her, consider whether these thoughts at the back of your mind were trying to tell you something - are you looking for a girl who's more adventurous? Maybe you want an open relationship? Explore these options before you end up going back to square one (having intrusive thoughts about fantasies but being unable to complain because you're torn between love and fantasy). It sucks, but think it over. And stop being sad, you didn't do anything wrong, it's part of human nature.