Dude, you banged her; mission successful. Had you known it was going to end up like this from the beginning you would have still taken it, right?

There are probably people out there that envy what you DID have with her and for sure if she does get back with her ex, he will always harbour some jealousy towards you.

You haven't lost out on anything really. She probably just invested loads in her ex. It's fucking really hard to win a bird over when she is still in love with another guy. You did well getting as far as you did.