See it as progress. You have recognized her behavior. You have named why she has the behavior she has. You have the ability to know she is QUACKING away. It is good she is good to your children.
Now all you have left is dealing with it. So she comes once a month. I always try to see things in short bouts as, "I can do anything for five hours, or one day out of 30 or whatever." This helps me see that while it seems to take forever at that moment, it is really just a short amount of time. Negative family I try to see as the same as my work. I work with customer service. There will be those I really like, and those that stink and I cannot stand. I still have to deal with them and not let them get to me, that is the right thing to do. Maybe you can see your mom as your customer that you have to deal with, not take personal, and move on from!
You are doing great Stung!!!!