What to do about her seriously?

I'm no expert or anything but believe me i've messed up before. You had the chance and didn't take it. I've done this and it sucks. Now that she's back with her boyfriend you'll have to think of a few things then go from there. Such as the odds that they'll break up again, why'd they break up in the first place, how much does she like her boyfriend then the kind of girl she is. Assuming you know most this stuff you'll be able to decide if you want to get with her anyway or if it won't last long and just wait. Good luck and try to keep her venting about him with you as low as possible.