I think breaking the news in the presence of your marriage therapist is the way to go.
It will provide the perfect opportunity for you to express yourself without interruption and for your H to actually HEAR what you have to say.
If, however, your plan is to break the news in the hopes of "finally getting his attention", likewise, it could work in your favor or it could be perceived as a cheap trick by H and he may not take you seriously later on.
Just be sure that you're actually making a true statement and not just looking to shake things up. Bluffing a separation and subsequent divorce is not something to toy with - be sure that when you make this statement, it is truly what you want the outcome to be.
Hugs and best of luck. I've been there.
