That's what fear and worry always seem to do; rob the joy of today by thinking about what might come up in the future and we often manage to magnify the "badness" of what might happen What you described sounds ... really understandable to me; it's great that things are going well and you had such a good session, but it makes things coming to an end with your T seem closer and more real than before and because your T is someone very important in your life it's very natural to be scared. Maybe something you should share with your T so that you can work on what will happen when things do wind down and how to hang on to the joy now and not focus on the fear?