I think it's nice that you're able to see your old t on an occasional basis during shared social events and behave normallly with her, saying hello and chatting a moment or two before moving on.

I also think it's good that you realize that part of your ability to normalize your relationship with Bt may be because you have a new t who you have become attached to. It's not a bad thing to switch your attachment from one to the other, as long as you recognize it as a pattern you have and seek to learn from it in your work with current t.