I think that because your mother is so intwined in your life, you likely would have had to say something sooner than most people anyway, and at that point it would have been because she "found out" something.
The fact is, you are an adult, and you and your husband have come to this decision together. I think it is time for a sit down to explain to her that while you value her opinions and the time you spend together as a family, that you are now an adult and she needs to respect that the choices you make may not be the choices she would make AND VICE VERSA. I think some people forget that their choices might not be "approved of" by all as well.
What did your therapist say when she showed up? Maybe you could have this discussion with your therapist there (if you feel like this would help), so that there's a third party to be objective.