<human>: Can you condense the text into a few key points?

I (32f) work closely with another woman (21f). I'll just call her Hannah. My administrator hired Hannah as my assistant to help deal with the extra workload. We've worked together for about 3 years and I've never needed to micromanage her or treat her as a subordinate. She is very competent, energetic and hardworking, and I delegate a fair share of work to her. She knows her usual responsibilities and works autonomously with occasional guidance most of the time.

She's always had a tendency to let power/responsibility go to her head. Lately it's becoming a more of a problem than just a minor nuisance.

Hannah has recently been going to some management/leadership seminars, *not* at our boss' request , mind you. She has begun to seemingly enjoy rudely mouthing off at clients, who we'd prefer to keep happy, obviously. Nobody has complained yet but I worry it's only a matter of time before she offends someone and they take their business elsewhere.

Just in the past couple of weeks this behavior has escalated to trying to order me around and also using obvious manipulation tactics to try and outsmart me and get her way. She has started demanding to know my exact schedule, texting me about work on my days off (with no real need) and trying to control details of my work.

Some people may assume from this that she's trying to steal my job, but she isn't qualified, and would need to get a different degree. So there's no risk of me losing my job to her, but I'm sick of her trying to push me around. She just seems to want power and control over people. How can I get her to mind her own business without getting rude or confrontational? Manipulation, passive-aggressiveness... all of that is on the table here. I don't want to lose a valuable person, but I can't carry on like this.
<bot>: A coworker who is meant to be my assistant has begun impolitely mouthing off to clients and trying to control me. The behavior needs to be stopped while avoiding a blow-out.