I tend to agree with her. If I want to vent 'feelings' I talk to my girlfriends. I've long since given up trying to do that with my hubby.
That being said, perhaps you are a guy who does understand girly feelings. That's cool. But it's not cool to have her venting issues about her relationship to another guy. I sense you have feelings for her which aren't so girly. If so, BACK OFF.
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.