Demi Moore is furious that Rob Lowe has chosen this moment to blab about their ’80s fling just when she’s preparing to say goodbye to her beloved ex-husband Bruce Willis, according to sources. Moore, 62, thinks it was “mean and even dimwitted” for Lowe, 60, to stir the pot now by revealing their long-ago hookup while making the 1985 Brat Pack flick St. Elmo’s Fire, insiders toldRadarOnline.com. Said a source: “She always knew Rob was a jerk and this just proves her case. Clearly, he’s desperate for attention. If he had half a brain, he would have known not to even go there. But then, everybody…