But can you really know for sure that separation from your T will be as terrible as you think it would be? If the whole reason you're in therapy is growth and insight and self-improvement (not that it is the reason for everybody, but still), then maybe you can be open to the possibility that Future You will feel differently about it. It will probably never be easy, but maybe eventually you will be in a different place where you feel grateful for what you learned, know you will miss him, but feel ready to be on your own.

It might also be worth asking your T how he handles termination so you can picture what it would be like. I terminated with my former T before I was ready because I was graduating and moving away, and I got a lot out of staying in touch with her occasionally via email, first every few months and eventually every couple of years until she passed away last spring. There was an element of growing up but not having to go away completely that was really nice. It was also so gratifying to share good life news (like sending some wedding pics) and have her respond with excitement that was rooted in her understanding of how I had struggled when I was much younger.

I "terminated" with my current T several years ago when she and I were mutually satisfied with how things were going with the issues I was working on back then. (I actually ran out of things to talk about, if you can imagine that!) I came back for new life stresses about a year ago, and at this exact moment I need a lot from her, but I can remember that I had gone years without seeing her when things in my life were going more smoothly. I know eventually I will be "done" again, but it's helpful for me to remember that she said she will have me back any time as long as she is still practicing and we both live in the same place.