I'm not sure I can help very much. I don't know you or your t, or much about your situation. There is obviously a lot of concern and anxiety around this. However, can you define whose insecurity it is? Is it all cominf from you, or is it also coming from the younger parts? You seem very reluctant to let the younger ones bond with her based on the fact that it is a temporary relationship. However, all relationships have an end at some point. The only person we take to the grave is ourselves. I understand this is a generalization, and that this may be shorter than you were intending. But I feel like it would be an amazing chance for the younger ones to learn about healthy relationships. How you CAN be attached to someone and feel close and supported and have it end (at some distant point) in a positive way. Terminating therapy is a process in itself, especially after years, so it wouldn't just be that one day it was over. I'm sure your t would take the time to help all the parts understand the meaning of the end of it.
And what a great learning experience for them, on trust and positive attachment.
Of course you're worried and upset. With your history (well, with any of our histories) it only makes sense that trust and intimacy would be an issue. especially if you never got the love from your mom that you deserved. Not because of anything you did, but because you are. You are you, you are human, you are. Period.
My question from earlier I guess is to see if it is the little ones who need the support through this, or if you are using that anxiety and fear of future hurts to make it more difficult for the little ones to get the support and love they need?
This isn't a question I expect you to answer to me, or to anyone really. Just something to ponder on your own. I find questions like that will sometimes help me to figure out what the right direction to go is, based on how I feel/think/react to the ideas presented. Only you know what is best for you. Even if you think you don't, a part of you will. Intuition, gut feeling, emotional response. Something will tell you if you are on the right track with it.

I've worked with, and been a client to many many mental health practitioners. The ones who connect with you, that is not fake. They actually care. The relationship is focused on the wellbeing of one person, yes, and it's unlike other, reciprocal relationships. But doesn't mean she doesn't care about you a lot, and it doesn't mean it's fake or fantasy. I would mention one thing, though. Mom's will always have issues with their kids, even the 'really great' ones. Every mom will make selfish choices, guilt trip, yell and be upset, be demanding, not understand and not put you first. Not all the time, but these do happen, and fairly often. You don't need a mom. You need nurturing. You need care, kindness and empathy, and unconditional support. A relationship with a t is a pretty good place to learn about those kinds of supports.
I hope it goes well tomorrow. Please keep me updated, I'd love to hear how it goes!
(sorry if that ended up sounding really preachy, that wasn't my intent!!)
xo
IJ