I think your email said how you felt. It was honest.
I think you're far ahead of her and maintaining control. It does get annoying with the things they say and do. Whether it be total ignorance of the wrong they've done or trying to get your attention thru childish ways. I think you are in a better place than her. Don't let her tactics get the better of you and drag you back down. Also I'd think about going to different therapy group. I would find it hard to heal in a place meant to heal when the person who's causing you angst is there. I know you've said you've been there for three years but she's obviously intruding on your space.
I regret replying to her email only because it continues the "toxic dance" between me and her. That's why No Contact is so important. She wants us to continue to argue and tear each other down until both our lives are destroyed. (Her's is a train wreck).
Giving up the support group sucks. Mainly because I'm actually diagnosed with Depression and it helps me immensely. Where as she has BPD and uses the group as a pickup joint. It's like her social playground. I may just have to stop going to the coffee afterwards. Because that's when she stares at me and paces around the table I sit at. The actual meeting itself divides up into 3 smaller groups. So I'm able to avoid her in there.
She could care less that her attending that meeting is driving me crazy. As usual, it's all about her. She's the most selfish person I've ever encountered.