For me it has been several things:
1) time - just showing up in my appointments, just the raw hours of time being together has a lot of potency
2) "Recognising" my T from the very first session - I did not compromise with my self when I decided to continue seeing him after the first time, I felt he was the right person for me.
3) He has never insisted anything on me, he has just let me be where I am. This has many times still been intolerable to me because if he lets me be then I feel abandoned but if he comes closer then I feel intruded upon. It has taken a lot of time to really carve out this pattern so we could both see it in moment.
4) In earlier years he let me call him (even every day if needed) and email (although he usually did not answer). He let me call and email over his vacations. Once he even come out from his holiday to see me because I felt I just can't wait anymore.
5) He has welcomed and absorbed a lot of my anger and rage that I was completely aware of before starting treatment

However, I am 5 years in, more than 800 sessions and I think I still have a long way to go.