I would not give up on the mare!!!! Just invest more time in your ground work, make more things than just lunging her around. What kind of exercises do you do. Maybe she needs to gain more confidence and needs to trust you more before she accepts you on her back, the best way to do so is ground work. And I mean diffrent things, not just let her go or run in a circle. Make it fun and make sure that she respects you. Try not to move to much when you stand in the middle, let her go around you without you moving your feet! Make her change directions in walk, trot, canter. Make her stop out of the canter and make her canter out of the stop. Make her back up and come into you but make sure that you invite her in to you and that she does not do that by herself without you asking. See how respectful she really is and act fast, correct the smallest bad thing as quick as possible and give her lots of time when she does something good! It takes time but I had a horse that was just perfect on the ground (after more than a year of ground training) and she would not even let me come close to the stirrups to mount. It took so much time to gain her trust and once I got on and she trusted and accepted that, it took about 2 trail rides and about 2 hours in the arena and that horse had like something switched in her brain and she started learing to respond and respect me on her back. She is now really well under saddle and a super safe horse to ride, BUT - it took about 2.5 years to get there.... Be patient and as you do not know what really happend to that horse just assume the worst and try to be understanding but firm. I am sure you will be able to enjoy your horse at the fullest soon!