Thank you, thank you, thank you! Very good advice ~ all of it. I will layoff when she is in heat, because she is definitely a whole other horse.
My other problem is that she does what I ask her to do almost immediately every time. I'm never ready to quit "playing" as quickly as she is... as I've said I'm new at this and I want to "practice" for more than 5 minutes... LOL! However, should I be moving on to something else once I get compliance?
I would. I find that most horses get soured on seemingly pointless exercises or repeating the same things over and over again. They aren't dogs and don't have the same interest in fairly repetitious "play" (and I doubt they see the backing and yielding exercises as play). If you're working towards something, whether it's ever increasing elevation and engagement for a dressage horse, or going round a course of fences for a jumper, or chasing cattle for a cutter, or even going somewhere for a trail horse, you'll keep their interest. Even then, a good trainer won't jump the same fence one hundred times, or spend an hour schooling only the half-pass on the same line.
I'm not one for drilling groundwork. If I can lead the horse from A to B and it's soft and willing, get it to yield its quarters and forehand and back up, have it stand tied (or even groundtied) for grooming, tacking up, and mounting, then I'm happy and I don't belabour that work. I've seen horses get cantankerous and sour when they have owners who make them do little circles or yielding exercises just to do it, but without any point from the perspective of the horse (horse already knows it, but owner thinks it improves the "bond").
I had a music teacher once say "practice makes permanent, not perfect." If you're "practicing" free lunging your horse and just end up chasing her around and getting her worked up, but requiring a second person to cut her off, you're not practicing the timing and feel, the skills needed to do that exercise in an effective or useful way.
If your MIL is horsey and gave you the horse, why isn't she helping you?