Welcome back misssouthernbelle
Mother's Day is difficult or awkward for many children of BPD parents posting here. I have an uBPD mother myself and Mother's Day has always been a day I struggled with, especially after experiencing some extreme BPD behavior from my uBPD mom & sis on Mother's Day a few years ago.
Healing from all of this can be quite tough, I am glad you are feeling stronger though and have had the support of a therapist.
You are also dealing with your stepmom who has certain issues of her own. Losing so many children through miscarriages is a huge psychological, emotional and physical blow. It is unfortunate that she so far refuses to seek help, hopefully that will change in the future.
So, I sit here on Mother's Day, after being invited last minute (yesterday morning) to my stepmom's family gathering, really pondering if I made the right choice. I sent her a nice text this morning, telling her it was the last week before finals and I have a lot to get done. I told her that if I had a little more advance notice, I could have probably adjusted my time accordingly. I wished her a wonderful Mother's Day and a good time on the lake.
I think you handled this very well

Especially considering you only got this invitation at the last minute.
I'm no longer bitter about it, but it's still a day that a light gets cast on the hole that is in my life. People need to respect that and respect what I've been through.
I am glad you are no longer feeling bitter about it. Still, growing up with a BPD parent is very tough and accepting that reality also means letting go of the fantasy parent and life you never had but deep inside might still have longed for very much. Especially on days like this it becomes very clear, sometimes painfully clear, how much we've missed out on and that we never really had a true mother.
Take care