I have the exact same issue of searching since early childhood for a mother figure. I won't go into detail here because I've written several posts about it. But I totally understand the struggle you mention, and I identify completely with your feelings and the way you've described the issue.

My unmet needs from childhood are the main issue in my therapy, and my ability to accept my t's caring and help without wanting her to be like a mom to me, is the hardest thing I have to face.

We have been working to instill nurturing motherly qualities inside me, but it is not going very well. I just don't "feel" motherly toward myself. It feels like I'm just faking it, more like a babysitter or something, I dunno. I agree that it feels heartbreaking to accept that maybe we can never get from anybody what we didn't get from our parents growing up. It is also hard to understand how to mother ourselves when we didn't get it to start with. We are trying to produce a safe, loving, stable feeling of protection and value inside ourselves that we have never felt. Like the blind leading the blind. Our t's can assist us, but most of them don't want to take on a mothering role.