Your girlfriend is still in grief over her previous relationship. If they were together for six years it will take (on average) about a year to get over it completely. Many people make the 'mistake' of launching into a new relationship before they are fully over the previous one. That's OK. Sometimes that can work out fine. But grief is a tricky process and it's important to remember, it's not about you. It's about her. It's about her feelings of loss that have not yet had time to heal over. In an ideal world you would have met her about a year after the break up, once she'd had time to process it all properly. But we don't live in an ideal world, we live in this one. So, this is the situation you are faced with. What to do?
If you really love her, just love her. It's simple. Just be there for her. Listen to her talk about the problem by all means, but you don't have to let it dominate your life as well as hers. It's not actually your problem to fix anyway, it's hers. If she starts talking about it a lot, just gently listen for a little while (without trying to fix it, make it stop or change the subject). Give her half an hour or so to just lament (whine, complain, reminisce etc). Then quietly change the subject. Then talk about something else. Something 'neutral' is good. Maybe put on a movie or some music or go out or do something else just to try to 'switch channels'. Eventually, if you keep doing this - first the listening then the 'something else' - the wound will heal. Then she will start to think (all by herself) "Wait a minute. What am I doing talking about this other guy that didn't work out when there is another, much nicer guy right here right now with me, loving me, being sweet and kind to me, taking care of me, being here with me, showing concern for me...' That's what women want. It's not really all that complex when you get down to it.
In short, give it some time. Be patient. If you really love her, just support her and show her you are a great guy. She'll figure it out soon enough.
"The only thing to do with good advice is to pass it on. It is never any use to oneself." - Oscar Wilde