How can I tell my boss that she is interrupting me ?...?

Sounds like someone I know. You need to get down to brass tacks here. Get serious now...sit down and draw up a plan, one thing at at time to be accomplished...none of this multi tasking drive you crazy nonsense. Explain the hazards as you have found them to be and include the mental chaos it creates when she leaves. She disorganized in her mind and she must come to understand she is causing you to be the same and you can't continue to work like this...and how it is counter productive. You need to be able to feel a sense of accomplishment at the end of each and every day. Issuing edicts and orders is the easy part but accomplishing tasks and goals is another matter with the pressure she creates. Be very diplomatic of course.

Task your manager with prioritizing your workload and her expectations. You will have to assume the role of ordering chaos.

You do not have to blunt or beligerant about it. Simply redirect her thought process on the current tasks and ask her how the new task takes priority over the existing ones.

You can use an anology like, you already have a helping of meat, potatoes, and a vegtable, which one will she be replacing since only so much will fit on your plate.

If this tatic doesn't work, step it up. Ask her if she feels the group is meeting their objectives, and where you fit in the mix. Is she getting the results she is expecting. If the answer is no to any of them, ask how she feels it can become a yes. Then speak to an instance where you felt she was hindering your performance.

If shes interrupting you again try to show her that your busy & theres so many things to do with this important one and you have handle this first kind a like that!