Just what did this flirting consist of? I'm wondering if you seriously crossed the line.
Also, were you and your girlfriend happy...or did you have 'ups and downs'? Seven years without getting married is a very long time....is there a reason you hadn't got around to marriage? Or, if one of you doesn't believe in marriage, was the other cool with it?
Sorry for all the questions - but background really does help.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.