Have you told her you are working to cry while in session? She can help :-)

As you pointed out there can be several reasons why the end happened like it did. I don't think she wanted you leaving upset is why she tried to lighten the mood a bit. It was the end of session and there wasn't anything she could do for you since the session period of time was up.

I use to get the end-of-session upsets and it is so hard and frustrating but eventually one learns to allow things to happen when they happen instead of putting them off. Eventually, when you can do all the work within the time limit of that session you will know you are done therapy. But containing one's feelings or responding in the moment are such complicated things to learn.

Your T didn't abandon you; start the next session with some of your feelings about feeling that way and what happened at the end of this session. I guarantee it will be helpful.