It’s not at all unusual for people who did not have a healthy relationship with their mother to find other people to fit the role. In fact, the dimensions of many female friendships have a “mothering” dimension to them. As long as there is reciprocity, it can be healthy.
Since you mention BPD as one of your issues, I do have concerns about you calling this woman “mom”. It may set up expectations that she can’t possibly meet. It may encourage a level of dependency on your part that doesn’t belong in a healthy friendship.
I suggest you work with your therapist on grieving the fact that your biological mother hasn’t been able to be the mother you needed. Then create a circle of friends who are mutually supportive. Do talk to your therapist about this exchange with me. A therapist who knows you can give you better advice.
I wish you well.