I cheated - I got a horse, not sound for riding, whose job it is to act as companion for the one left behind (for a total of four horses).
Perhaps you might want to consider a companion. It wouldn't have to be a horse; it could be a donkey, pony, goat, etc. Feed wise, if you get a smaller animal it doesn't add much to the bill.
Otherwise, its practise, practise, practise at home before hitting the trail. Perhaps do quite a bit of desensitizing of the mare using tarps, flags, etc. and that may get her used to having her space invaded. One trick I suggest when ponying on the trail (assuming you're using a western saddle): Wrap the lead rope around the horn (one complete turn) and place the end of the lead rope between the saddle and your leg; this allows you to use two hands on the reins while your leg holds the lead rope in place; this is useful if you need to get more control over your riding horse; if things get really out of control and you want to let the ponied horse go all you have to do is release your leg pressure and give a quick reverse turn of the lead rope and it's free.