Dear Cindy: It seems from your question that there are really three issues here.

First: your aged mare, whom you have had for many years, has been living quietly with a filly you bought four months ago. She has become very aggressive now that two new mares have been introduced into her space. One of these mares, it seems, is the mother of the filly. Horses are highly social animals with a strongly hierarchical society. In the wild it is usually a mare that dominates the herd. She is submissive only to the stallion. When a new horse is introduced into an established herd everyone has to figure out where he or she now stands in the suddenly new pecking order. This is usually accomplished without undue violence, and once the new horse knows his place, everyone is happy. Occasionally you will find two dominate horses who will continue to fight for leadership, with neither of them backing down. I have also known horses to be seriously injured because the boss horse would not accept them as part of the herd. So some care is needed when introducing new horses into your herd. It is a good idea to let the horses get to know each other over a fence before putting them together.

It sounds as though your original mare is anxious to establish her dominance over the others and she may feel especially threatened by a bond between the filly who has made up her herd for the last few months and one of the new mares. I would encourage you to separate the new horses from your mare and filly if possible. Is your pasture large enough to divide? Just having the new mares on the other side of a fence may take enough pressure off of your mare to allow her to calm down. If she continues to be dangerously aggressive toward the filly even after the new mares are removed, you may need to think about putting the filly on the other side of the fence as well. While aggression in a herd usually sorts itself out fairly quickly, occasionally it can be a serious problem and you may need to try several different arrangements before you hit on the one that works

Second: your original mare, age 19, is now hyper and you have given up riding her because at times she acts wild and has thrown you. Hyper, flighty horses are using the reacting side of their brains, not the thinking side. To encourage her to start using the thinking side I suggest that you do some intensive ground work with her. You can find descriptions of some of these exercises in my article, A Good Grounding. Ground work will establish you as the leader because you will be controlling her feet, her direction of travel, and her speed. Your mare will be encouraged to stop reacting and start thinking, and when that happens she will become much calmer. Remember: horses are emotional animals, so when you are working with her you must stay very calm and unemotional. If she pitches a fit, simply ignore it and continue with the lesson. If you get mad because she is mad, she will just get madder still and then you're both on a downhill spiral. Be sure you are working with a long enough lead line that you can stay safely out of the kick zone.  You will need to continue each ground work session for as long as it takes for your horse to show signs of calming down. She will lower her head, begin to walk and trot calmly, and will show signs of relaxation and submission. Only then should you end the lesson. The most common mistake people make when doing ground work is to stop way too soon.