Evidence I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'erleaps itself And falls on th'other -- / [Enter Lady Macbeth ... to keep power for himself ...

Lady Macbeth is still concerned they act, and cope, as a couple. This is important. Remember she can only really have power through her husband. She has no power of her own. So her main worry is ...