Reading your post reminded me of when I went thru that stage with my mum, like most people do, where I just realised one day that she is not immortal and one day she would die. I was very young at the time. I remember waking up at like 3 in the morning and rushing into her and clinging onto her because at that moment I realised she would not be there forever. And i was crying my head off, lol.

They say in therapy sometimes you develop this attachment to your T (if you didn't get the opportunity to with ur own mum) and so it would follow that you would have similar phases to the one i described. I was thinking maybe you are worrying about your T dying for a similar reason? It's a bit like you always KNEW they would die, but at that moment the thought is thrust into the front of your mind and it feels like its happening right now, or could happen at any time, but its not. Its just sort of a realisation.

I dunno if this fits with what you described, its just something I was pondering when I read your post hehe