OP - I think the problem is as much you as it is your staff.
You call her a 'difficult Direct'. You talk in business jargon and have a worrying tendency to overuse "quotation marks". You hard ball her over things she deserves. Given that you had a previous relationship with her, I think there is a lot of control issues being played out here
You have feelings for her. Unrequited maybe. When detailing your age, you say "mid to late 20s". How odd. You get very defensive when advice is given that doesn't match the videos you have watched or the books you have read. You continually write of being a good "leader".
From your posts on here at least, I think you would be a very difficult manager to work for.
My only suggestion, as I guess I should be constructive, is to start treating this person as a human being and not a middle management case study. The best managers I ever had, and the ones I really learned from, knew how to treat people as people, and didn't try so hard to be a "leader" (your words, not mine).