I agree with everything contained, but you mention business discussions are your main reasons to communicate, so are you in a position to buy your partner out so you can make a clean cut?

It sounds like your ex can't keep things from the past seperate from the present day situation and if he can do that, then try (or you may be forced to) to separate the business before it becomes too disfunctional and the business fails and is of little use to either of you.

Just my two cents

Shannon