This is just me guessing, I don't know either of you, but I think maybe these things didn't just 'not matter' to your T. Has he said that it didn't matter to him, or that he doesn't feel anything? If so, then of course this is probably the wrong guess. But if he hasn't said anything like that, I think it can well be that it did matter to him and that he did and still does feel things. But usually, Ts have learned to manage their emotions well. While they like to work with their clients, care for them and everything, if one of them leaves, they can't just break down as though they'd lost their best friend. It would not be healthy for them to feel this way. That doesn't mean they don't feel anything, but they feel a different kind of connection to their clients than the other way around.

Now your T has decided that he wants to not be a T anymore, for whatever reason. He knows how to deal with the losses of his clients. He has decided to do this, so he's probably already happy with his decision. And on top of that he has probably experienced a lot of terminations. You on the other hand have probably experienced a lot of trauma related to being left alone. And this man was a person you socialized a lot with and you on some level trusted to 'always' be there. Your grief of the relationship will look and feel different than his. That doesn't mean he doesn't grief it, but since it's different, he can easily say he doesn't want to continue being friends or something similar, while for you this is hard.