A certain topic just could never become water under the bridge for Kristen Stewart and Robert Pattinson. 

The world was shocked to learn this week that things had gotten so bad between the two that they split up. So what was the problem?

Apparently, they were still arguing about Kristen's fling with director Rupert Sanders from last summer, along with other major issues, according to Us Weekly.

“All this stuff came up,” a source says about their recent fights. “A lot of issues came up.” 

This included arguments “about the cheating, about how selfish she always is, about how moody she always is,” the source adds, with particular focus on the fact that she cheated on him.

Long story short: "Rob is torn," the insider concludes. 

Wow. It's certainly tough that Kristen is still getting punished for something she did almost a year ago, but it's understandably a tough event for them to move past.

We hope they can work things out yet again, but it's not looking good. Sigh.

Source: Us Weekly