Hey Hopeless419, welcome to the boards

Glad we can be here for you as you cope with your SD28. Stepparenting is already hard, and adding BPD behaviors in the mix doesn't make things better

One of the weirdly nice things about this group is that the challenging person in your life doesn't have to have an "official" diagnosis of anything, for you to get support and skills here. There's a sense in which her label or diagnosis isn't as important as the fact that whatever anyone would call what she's doing, what she's doing is stress-inducing, dramatic, dysfunctional, and crazy-making. We can work with that. It's the behaviors, not the labels, that bring us here.
These parts of your post jumped out at me:
I’m exhausted with trying to help and constantly being the bad guy.
and
She actually turns 28 this week and my husband will take her to dinner but I won’t go. I just can’t handle being around her, my anxiety skyrockets. Is this normal when dealing with someone like this?
It sounds like you've shouldered a lot with trying to help SD28 "launch", and you're at a pivot point -- there's not much more you can take.
What would it be like to do more of what you're doing for the birthday this week, where you decline to participate, and give yourself a break from interacting with her, or feeling responsible to "be there for her"?
Do you think your H would understand?
Keep us in the loop -- we'll be here;
kells76