These are great questions! I wish I knew the answers; I hope you get some helpful responses. I can share my experiences but each of is different, and each T has a different approach.

From my own experience, I think that I'm mourning the unmet needs every time I get triggered by something I want from my T that she can't give me. I'm mourning right now by crying because she's away from me. I'm sure that's not all about her in the present; it's about my unmet needs from the past.

Ts can help by exploring what these needs are and how you can help meet them yourself. In IFS therapy, you talk about those parts that didn't get what they needed back then, you listen to how they feel, and then you discuss how your "adult" self can give those parts what they need from you, like hugging, touching, playing with them, and telling them you'll always be there to take care of them. In essence, your adult self CAN help meet those needs now.

My T helps me by actually meeting some of those needs though it can never replace whatever I didn't get, of course. When she holds my hand it's healing for me. It gives me a sense of safety I never got in my past.

So I think it's a combination of feeling those needs in a safe place, with your T, crying about them as much as you need to, just like mourning a person who dies, and accepting that you can't totally replace what you never got. Then, meeting some of those needs yourself, or allowing others, including your T, to give you some of what you needed. Just by being there for you consistently a T can help satisfy those needs.

I'm looking forward to the responses in this thread.