Groundwork doesn't mean just lunging. For instance, I do lots of groundwork, but I almost never lunge! We work on respect, personal space, connection, light cues, responsiveness to cues I later use in saddle, general intelligence, trust, facing scary situations and just spending time together. Hillwork can also be done from the ground, as can be cavaletti work, working at liberty, and you might be interested in some techniques from Clinton Anderson or other reputable natural horsemanship trainers, that can help keeping your groundwork interesting, enjoyable and beneficial for your horse. Also, if your mare is barn-sour, that already is a big issue you could be working on! Starting with her accepting you as a true leader, because a horse is always ready to follow the leader of the herd, and a rider with a horse forms a mini herd. If the horse is not eager to follow the rider in a scary situation, one of them might not really be the leading horse... ;) For starters, you could hack in the trails by hand - and later transfer it to saddle.
Also, spending some more time to actually soak beet pulp than giving her grain with pulp in it might really benefit her health, excess energy in particular. Again, it is really like giving a kid junk food, because it takes less time than making proper dinner at home. I am not saying you're not taking good care of her - you obviously love her and care for her very much! Thus learning more about horse nutrition might help you in the way of caring for your horse.
I have come a long way, to surrender my shadow to the shadow of my horse.