That's a difficult and, unfortunately, not uncommon situation. You should have a frank, unemotional discussion with the past president. Let her know that you understand she's trying to be helpful, but some of her actions are making it difficult for you to establish yourself as the group's leader. It's likely that she doesn't really realize that she's undermining you. If you point it out in a positive way and without being accusing, chances are you'll see improvement in the situation. See if you can find a role for her where she can still use her talents but won't be in the middle of the action. Maybe it's the two of you meeting for coffee every couple of weeks to discuss what's going on rather than her attending meetings, for example. Before you have this meeting with her, think about ways she could help without becoming a distraction. The article How To Retire Gracefully
is aimed at retiring presidents, but it might give you some insight into what specific actions are causing the problem and how to address them. Good luck!