I feel EXACTLY the same. It's a very common experience in therapy, especially with a painful background like you sadly describe.

I'm not sure people ever really overcome it. I haven't. What has helped me is to be honest about it and talk about it all with T. It is in being open about these feelings that I started to move through them and feel the grief over not being able to have the mother and family I want. It also meant I could start grieving for my childhood since I am more aware of what loving care and a deep emotional connection (from my T) feel like and what I didn't experience with my mum when I should have. Unfortunately, many therapists are uncomfortable with maternal transference and seem to want to skirt around it. I saw several therapists who reacted that way - they didn't want to hear how much I longed to live with them, be with them constantly, be held, mothered etc. But the healing is in putting voice to these feelings and my current T responding with compassionate acceptance and understanding and sadness for my grief and longing. The feelings don't just vanish but they get easier to deal with and eventually, as I'm finding, there are signs that the grief is less intense even though it will likely always hurt.