grounding would be a good first step if you have not already attempted that. The PTSD forum here has a sticky on it I believe. Also, working on coping skills to get you through a flashback, and working on staying safe during them are also some things I have been taught.
They can be scary and all-consuming. As someone mentioned above, they tend to get a bit worse before they get better, and dealing with triggering things in therapy can certainly set them off. I go through them in cycles it seems, and they often get worse for a time when we tackle really difficult things in session. I have found traditional grounding is very difficult for me, so I ask all new T's to engage me in cognitive tasks to help me get grounded. My flashbacks tend to be more physical though...
Sorry you are going through this.

One thing that has helped with my flashbacks has been to sit through them reminding myself that it is just a flashback, and not happening in real-time. Again, this is for the more physical ones though. The emotional ones leave me a total wreck...
Can you try relaxation before session, or before talking about difficult things? Also, have you tried creating a "safe space" for yourself when things get rough? this can be through guided imagery or meditation... I prefer the beach in cape cod... then imagine going there whn the flashback starts.

Some other grounding that has helped me has been:
-repeating a mantra about my age, the date, who I'm with, where I am, reminding myself it's a flashback and not currently happening.
-talking about my dogs, but sometimes T has to ask questions to get me going.
-looking at T directly and trying to focus on being there instead of the past
-bring a stress toy or soemthign to fidget with
-I color or draw in therapy often because the slight bit of distraction allows me to be able to talk about difficult things while also remaining grounded.

more traditional grounding:
-engaging the major senses: touch something and describe the texture & feel of it; describe the room you are in to the smalest detail; hold an ice cube and concentrate on the feel of the ice cube; sniff something with a strong scent - peppermint is said to be really good for a quick "yank back to reality", and lavender for soothing; eat somethign with a strong flavor - really sour, something spicy, somethign bitter...