Do not, under any circumstances, send that email! You don't want to remind her that you are hurting.
I personally would recommend telling her you're busy that night. Let her wonder what's on your social calendar that you're not at her beck and call. Be mysterious.
If she really wants to "catch up", she'll try to reschedule for another night. If she just wants to spring more divorce financial bad news on you, she can email you.
You don't sound to me like you're in the right frame of mind yet to go and put on a strong game face. Better to wait until you're sure you can.
