Hello stepmom123!
This is a difficult and painful situation to be in, I can understand how you feel... .When you got married, I am sure you did not foresee what was going to happen down the road, and now you find yourself attached to a family member who you'd never even choose to be around, and want to protect your own child from... .
And yet, she is your husband's daughter - a most precious relationship. You love him, he loves her, and I am sure sometimes the push-pull struggle may even seem like there are three people in your marriage.
But I think you are on a very good path already - you realize what's going on and understand that you need to create some stability and structure for yourself and your own daughter.
When I married my husband, I never imagined the chaos and pain we would go through because of his adult daughter, and it has not been easy. But with lots of love and hard work, we have gotten through it all together, and today I would do it again (maybe a bit differently here and there with the wisdom and knowledge we gained through this process)

It isn't an easy path to take and it will require a strong commitment and work from both of you, but I want to encourage you: if your relationship with your husband is worth it to you, it can be done.
Boundaries are important, and there will be some basics that are non-negotiable for keeping your own daughter's physical and emotional safety, and also for keeping your new family's stability. Then, there will probably be many many other boundaries that will always be a "work in progress" so to speak, and on those, you will need to learn to listen to each other, support each other and forgive each other when the heart-strings pull you in opposite directions... .
I have a couple of questions: How old is your own daughter? Also, is your husband aware of BPD and/or willing to learn new skills and change his approach?