Taking you at your word that you can't go NC, then if you've already emotionally detached there may be little else you can do.
In theory if you are dealing with a toxic person, you can put up boundaries and it will go one of two ways: they will tantrum and then finally accept it, or they will tantrum and never accept it.
In the first case you're in for a rough ride but can eventually find a high-tension modus vivendi. In the second case nothing is going to improve matters because nothing you do is going to improve their behaviour.
In principle at that point the best you can do is give her as little information about yourself as possible, keep conversation as general as possible or about her, and keep meetings short and infrequent as is feasible.
Find ways to let off steam afterwards and give yourself time and space to cry or be angry, whether that's talking to someone, exercise, watchign a film or whatever. Really toxic people manage to elicit distress really rather effectively.
Caveat: if you have children then you have a duty to put them over your parent. Children are vulnerable and destructive adults ... destroy. If that means putting yourself first over her needs, then that's what you need to do. Dealing regularly with a toxic influence places massive strain on you, which your children tend to pick up on - something that people who can't bear to go NC often try to deny. In a way it isn't about valuing you, it's about protecting your children because over stressed and ground-down adults are less good parents than normal-stressed parents.