(((Willowleaf)))

I have had the same pattern. I can cope absolutely fine while my T is away, be very happy for her opportunity to travel or be with family, but then when she returns is when I have the issues. Very much like you, I would be disappointed and frustrated after sessions when we resumed. Or I would be very withdrawn in the sessions.
Like you and your T, we have talked about it and explored the possibility of repressed anger/disappointment about her going off and leaving me, fear of losing the connection, and even a combination - the fear of losing the connection causing me to deny the anger/disappointment. Also, the idea of withdrawing as a self-protective behavior, to avoid getting hurt like that again. Also the idea of withdrawing as a passive retaliation.

It's exhausting and disappointing to have happen when everything seems to be going smoothly