I'm glad to be here for you. It can make coping so difficult if you don't have a way to express how you feel and are stuck alone with your thoughts.
I can understand your feelings about being tethered to your ex all this time. His behavior sounds intrusive and demanding. And to always have to respond would feel like you never have any rights or boundaries.
And being in contact this much would also stir up your feelings each and every time, making you feel re traumatized over and over again.
Since you have to deal with him legally, one of the best ways to cope is to try to distance yourself emotionally from him as much as possible. Try not to give him any control or if you have to give him something, make it as difficult as you can. That way, it provides negative reinforcement to him. In other words, it won't be easy and pleasant for him to try to get something from you. It will be hard and difficult, making him think twice before going through it.
Grounding yourself during flashbacks is one of the best ways of getting yourself to come back from them. Here are some you can try:
Touch- grab something that is cold or has texture. It will bring your mind to the present.
Smell- keep herbs or something like cinnamon nearby. The smell will be strong enough to stop your flashback
Sound- use music or something loud to snap you out of the flashback
Taste- hot coffee, ice or even a lemon can bring your attention back.
Also, keep track of what might have triggered your flashback and also when, where and what time you experienced the flashback so you can learn ways to control them and eventually eliminate them.
By using these techniques, you can help yourself and have some control over how you feel.