You don't have to be the grown up about this. It's what I did when my T said he was leaving - I was all mature and composed in sessions, and then not so composed outside of sessions, and now I still have all these feelings - welling up and spilling over.

I could not trust myself to get through the unbearable feelings so I tried to not have them. I didn't want to make a big immature display of feelings in front of my T, I didn't want him to remember me like that.

Please know that your experience now is helping me - I feel these difficult feelings with you and walk with you, hoping to know a better way on the other side. Yesterday in therapy with new girlT, I talked about some of my feelings, and I talked about your situation, your not wanting to make it hard for your T. It helped me explain how didn't want to make it hard for my T - he was leaving, there was no changing that fact, and I could be mature and let him remember how composed I was about the parting.

I think our feelings in therapy are always pretty adolescent, juvenile, so I'm not surprised to read you are feeling immature.
Tell T how much you are wanting to take care of her feelings, wanting to make the parting easier for her.
She also needs to know you cannot imagine how you'll do without her, because then she can help you figure out how it is possible.
40 is enough. Time to come out of the desert of pain and suffering and get some care, comfort, sustenance.